Motive is something — a need, a desire, an interest — that moves a person to act: the inward cause of a deed, as distinct from the deed itself.
The word descends from Latin movere, "to move," through Medieval Latin motivus, "moving, impelling," and Old French motif. It entered English in the late fourteenth century in a now-obsolete sense, "a proposition or argument brought forward," and acquired its psychological meaning — the mental state or force that induces an act of volition — by the early fifteenth. The adjective is of the same age and kept the literal reading: motive power and motive force name whatever actually produces motion, and locomotive and automotive are built on the same stem.
Motive sits among several near neighbors it is regularly confused with. An intent is what an actor means to accomplish; a motive is why they want to accomplish it. An incentive is external and offered by someone else, which is why the split between 📝Intrinsic Motivation and 📝Extrinsic Motivation tracks so closely to where a motive originates. Criminal law draws the sharpest line of all: intent is generally an element the prosecution must prove, while motive is not — it makes a crime intelligible without being required to establish it. And motif, the recurring figure in music or design, is the same French word borrowed a second time and specialized.
"Motives are symptoms of weakness, and supplementation for the deficient energy of the living principle, the law within us." ~ Coleridge, Aids to Reflection, 1825