Despite widespread use in the literature, there seems to be little consensus on what the term ‘carbon (C) sequestration’ means. We differentiate between endogenous C, which fluxes within a system, and exogenous C, which fluxes between systems. Here we define ‘endogenous C sequestration’ as occurring when C fixation to release ratio is greater than one (fixation(a,s)/release(a,s) >1), expressed at the briefest, annually (a) and budgeted within a specified system (s). We distinguish between sequestered C (stored for >1 year) and temporarily utilized biologic C (i.e., labile C present within a living organism), developing equations for herbaceous and woody plant systems. Standardized expression of C sequestration with incorporation of descriptors, for example ‘somatic C sequestration(10 year, forest)’, clarifies the location, timescale and system being considered and should allow for increased transparency and improved communication for climate change debates and C budgeting.