Ion cyclotron motion in a strong magnetic field, recorded as an image-current transient and converted by Fourier transformation into a frequency-domain mass spectrum, defines Fourier-transform ion-cyclotron-resonance (FT-ICR) mass spectrometry. Trapped ions orbit at a cyclotron frequency inversely proportional to their mass-to-charge ratio; broadband excitation and detection of that motion yield ultra-high mass resolving power and mass accuracy ^[1]. The same trapped-ion population can be activated by collisions or by electron capture, so that tandem spectra report connectivity without leaving the ICR cell ^[2]. Conceptual boundaries exclude quadrupole, time-of-flight, and Orbitrap Fourier-transform instruments that do not use cyclotron motion in a homogeneous magnet, although all Fourier-transform mass spectrometers share the time-to-frequency conversion step. Essential features are a high-field magnet, an ion trap that supports coherent cyclotron motion, and a Fourier-transform data system. Exact-composition assignment in complex mixtures and localization of noncovalent ligand sites are analytical consequences of the mass accuracy and tandem capabilities inherent in the ICR measurement principle ^[3]. An FT-ICR spectrum of dissolved organic matter is correspondingly interpreted as a catalogue of exact molecular compositions rather than as a chromatographic retention map ^[4].