intrinsic means belonging to the essential nature or constitution of a thing — and, in anatomy and medicine, originating and included wholly within an organ or part.
The general sense describes what a thing is in itself, apart from its relations to anything else. Intrinsic value is the worth something has for its own sake, as opposed to the instrumental value it has as a means to something else; an intrinsic property, in philosophy, is one a thing would keep even if the rest of the world were different. The anatomical sense is literal rather than abstract: the intrinsic muscles of the hand or the tongue both originate and attach within that part, while extrinsic muscles reach in from outside it.
The word came into English in the late fifteenth century from Old French intrinsèque, from Late Latin intrinsecus, "inwardly, on the inside," built from intra ("within") and secus ("alongside"). Its early English sense was simply interior or inward; the now-dominant sense, belonging to the nature of a thing, is recorded from the 1640s.
Intrinsic is defined most sharply against extrinsic, its exact opposite: what comes from within versus what is applied from outside. Psychology rests a whole theory on that pair, contrasting 📝Intrinsic Motivation, behavior driven by the satisfaction of the activity itself, with 📝Extrinsic Motivation, behavior driven by rewards or the avoidance of punishment; 📝intrinsically motivated learning applies the same split to study. Intrinsic also sits close to inherent. Both describe qualities that are part of a thing's nature, but inherent stresses that the quality cannot be separated from the thing, while intrinsic stresses that it does not depend on anything outside it.
The word matters to me mostly through motivation. My own experiments taught me that rewarding creativity suppresses it, and that the expectation of a reward squelches it (see 📝Evaluation Confines Us).