【Stimulus】 , 【stimulant】 ,  【excitant】 ,  【incitement】 ,  【impetus】  can all mean an agent that arouses a person or a lower organism or a particular organ or tissue to activity.

Only the first three words have definite and common technical use.

【Stimulus】 , in this use chiefly a physiological or psychological term, applies basically to something (as a change in temperature, light, sound, or pressure) that occurs in the internal or external environment of an organism, is perceived by sense organs, and if sufficiently intense induces a neural or equivalent (as tropistic) response.

【Stimulant】 , typically a medical term, applies chiefly to a chemical substance and especially to a medicament that does or is intended to vitalize bodily activity, either generally or in respect to a particular system or organ or function.

【Excitant】  can come very close to  【stimulant】  in some of its uses; thus, one may speak of a substance as a  【stimulant】  or an  【excitant】  of intestinal motility. But distinctively  【excitant】  can apply to either a sought or an unwanted reaction and it can imply, as  【stimulus】  often but  【stimulant】 rarely does, either the initiating or the vitalizing of a process or activity. In their more general use these three terms are seldom as clearly differentiable as in their basic use.

【Stimulus】  and  【stimulant】 are usually interchangeably applicable to whatever exerts an impelling or invigorating effect (as on a process, an activity, or a mind), but  【excitant】 , here too, is more likely to suggest an initiating and it is applicable when unwanted or undesirable ends result.

【Incitement】 applies to something that moves or impels usually a course of action; the term tends to emphasize an urging or pressing intended to drive one into moving or acting quickly rather than the result attained.

【Impetus】  (see also SPEED 2 ) usually stresses the stimulation of an increase in the momentum of activity already initiated. But the term sometimes applies also to a 【stimulus】 that initiates action.