A @S05900@ @P04404@ @G02705@ (typically a nitryl radical) that is attached to a part of a @M03986@ whose microscopic environment is of interest and may be revealed by the electron spin @R05326@ (ESR) spectrum of the spin @L03424@. When a simple @P04404@ molecular entity is used in this way without covalent @A00512@ to the molecular entity of interest it is frequently referred to as a 'spin probe'.