The terms sustainable and sustainability are currently often used in scientific journals, including Energies. There are cases where these terms are defined or operationalized, but more often they are not. This is problematic, as there are reportedly hundreds of (different) definitions and operationalizations (in terms of standards or goals) of sustainability. This large number has its roots in history. Many current definitions and operationalizations of sustainability are social constructs. As these constructs vary, there can be variation in the characterization of specific ways to provide energy as sustainable or not sustainable. There are also definitions of sustainability that have emerged from the sciences. These definitions can also lead to differences in the characterization of specific ways to provide energy as sustainable or not sustainable.
Salas-Zapata et al. 
studied the usage of the terms sustainable and sustainability in scientific publications (found in the Science Direct and EBSCO databases for the year 2013). They noted that these terms were often (in more than 90% of the publications) not defined. In Energies
, there are examples of sustainable and sustainability being defined or operationalized, but apparently more often they apparently are not. The absence of definitions or operationalizations is remarkable, as there are reportedly hundreds of (different) definitions and operationalizations (in terms of standards or goals) of sustainable and sustainability 
. The large number of definitions and operationalizations has its roots in history.
2. A Very Brief History of the Terms Sustainable and Sustainability
Old definitions of sustainability refer to an equilibrium relation of humans with the environment 
. In 1713, H.C. von Carlowitz, active in German forestry, suggested sustainable use (nachhaltende Nutzung) of forestry resources: not to harvest more wood than is added to the forest stock of wood by the growth of trees 
. Since then, the application of this type of sustainable use to forestry has been geographically widened 
. In the 19th century, economists discussed the stationary state: an equilibrium between, on the one hand, the environment and, on the other hand, the economy 
. The economist D. Ricardo felt that the stationary state was ultimately inevitable 
, and the economist J.S. Mill stated: ’I sincerely hope for the sake of posterity that they (the people) will be content to be stationary, long before necessity compels them to it´ 
. In 1973, H.E. Daly 
revived this with his publication of Toward a steady state economy,
using ´steady state´ as a modern equivalent of ´stationary´. The focus of the terms sustainable and sustainability on an equilibrium relation between humans and the environment remained until 1980, when the International Union for the Conservation of Nature (IUCN) published the World Conservation Strategy. Living resource conservation for sustainable development,
advocating the conservation of ecosystems and biodiversity 
Definitions of sustainability exclusively regarding the relation between humans and their environment have persisted in the scientific literature since 1980. In the 1980s, operationalizations of sustainability in terms of environmental utilization space within boundaries were introduced 
. By now, a considerable number of operationalizations using sustainability-related boundaries have been published 
. An influential example thereof is the safe operation space for humankind 
. This space is defined within planetary boundaries as global environmental limits to avoid risking collapses of ecosystems 
. The environmental issues for which planetary boundaries have been proposed are in Box 1
. The safe operating space for humankind 
has been applied in ‘absolute sustainability assessments’ 
Environmental issues for which planetary boundaries, leading to a safe operating space for humankind, have been proposed 
Climate change: atmospheric CO2 concentration; energy imbalance at top of atmosphere.
Ozone layer depletion.
Atmospheric aerosol loading.
Fresh surface water and groundwater consumption.
Land system change: amount of forested land remaining.
Biogeochemical flows impacting phosphorus (P) and nitrogen (N) cycles.
Change of biosphere integrity.
Novel entities: hazardous human-made chemicals, plastics (tentative)
As can be seen in Box 1
, no boundary has been proposed for the usage of agricultural soils or for mineral resources generated in slow geological processes, whereas both are important for the functioning of current societies 
. This is in line with the focus of the safe operating space for humanity on preventing the collapse of ecosystems 
Agricultural soils and resources generated in slow geological processes are included when sustainability is defined as conserving natural capital for transferal to generations living in the future 
. So defined, sustainability is a characteristic of a steady-state economy 
. Currently, natural capital is defined as the stock of environmental assets from which products and services can be derived that are useful to humankind, now and in the future. It comprises natural resources (e.g., fossil carbon compounds), ecosystems, generating ecosystem services, and the physical environment providing, e.g., wind for wind power 
. Daly 
and Ekins et al. 
have stated that when the creation of substituting renewables at least equals the depletion rate of natural resources generated in slow geological processes, this can be considered to be conserving natural capital. However, it has also been argued that to conserve natural capital for transferal to generations living in the future, depletion of resources generated by slow geological processes should be near-zero 
In the 1980s, besides environmental matters, other elements were also included in definitions and operationalizations of sustainability. For instance, Solow 
defined sustainability as conserving the sum of natural capital (monetarized natural resources) and manufactured capital per capita. However, the most important change concerning the inclusion of matters other than the environmental came in 1987 with the publication of Our Common Future 
. This addressed environmental, economic, and social concerns. In Our Common Future
, it was stated that `sustainable development meets the needs of the present without compromising the ability of future generations to meet their own needs´. This definition is a social construct: a result of deliberations in the committee authoring Our Common Future.
Our Common Future
has started a proliferation of definitions and operationalizations (in terms of standards and goals) of sustainable and sustainability that are social constructs (outcomes of social interactions such as deliberations and negotiations). Such constructs often include environmental, social, and economic elements. Whereas the definition in Our Common Future
is general, operationalizations are often specific; they refer to or instance specific ways to provide energy (e.g., hydropower), specific activities (e.g., coal mining) or specific products (e.g., biofuels) 
. It may occur that environmental issues are not included. The paper of Dauenhauer et al. 
, who evaluated the sustainability of community-based photovoltaic projects in Malawi, is an illustration thereof.
An important operationalization, including environmental, social, and economic elements, is found in the Sustainable Development Goals of the United Nations. The most recent set of these goals is set for the year 2030 and numbers seventeen, covering a variety of environmental, social, and economic issues. For energy the goal (number 7) is specified as: ensure access to affordable, reliable, sustainable, and modern energy for all. Specified goals under this heading are in Box 2
. The history of Sustainable Development Goal 7 (which may be framed as sustainable energy development) has been outlined by Gunnarsdottir et al. 
Energy goals of the 2030 Agenda for Sustainable Development 
By 2030, ensure universal access to affordable, reliable and modern energy services.
By 2030, increase substantially the share of renewable energy in the global energy mix.
By 2030, double the global rate of improvement of energy efficiency.
Many operationalizations of sustainability and sustainable development as social constructs (sustainability standards, sustainability goals, sustainable development goals) have emerged from companies and groups of companies 
. They tend to diverge due to the variety of social contexts from which they emerge. Oil and gas companies have formulated divergent sustainability goals 
. The Mining Association of Canada developed the Towards Sustainable Mining (TSM) standard, applying it e.g., to tar, sand and coal mining. The TSM standard has been adapted by several mining organizations in countries outside Canada 
. The Minerals Council of Australia developed its own Enduring Value sustainability standard for mining, including coal mining 
Non-governmental organizations have also developed (diverging) sustainability standards, for instance for biofuels 
and energy in buildings, the latter linked to different definitions of sustainability 
. Governments of countries have been active in the establishment of mandatory and voluntary sustainability standards, the implementation and adaptation of sustainable development goals and the characterization of activities and products as (un)sustainable 
. They also have developed sustainable development plans for specific parts of the energy sector such as coal mining 
. The outcomes of these government activities diverge 
. There are variable definitions of sustainable cities 
. City governments have developed or adapted sustainable development goals in divergent ways 
and have been active in the fields of sustainable building standards and of sustainable energy action plans, with variable operationalizations of sustainability goals 
Furthermore, operationalizations of sustainability as a social construct have emerged from international organizations of stakeholders and experts 
. Examples of such organizations are the Roundtable on Sustainable Biofuels 
, The Forest Stewardship Council (relevant to forest-derived biofuels) 
and the Hydropower Sustainability Council 
. Another example in this category is the Sustainable Nuclear Energy Technology Platform 
, which operationalizes sustainability in terms of safe, reliable, and efficient. Sustainability goals for geothermal energy have emerged from a project involving experts and stakeholder groups in Iceland, New Zealand and Kenya 
All in all, the variations as to social contexts in which definitions and operationalizations of sustainability and sustainable have been constructed have led to variations in definitions, standards, and goals.