The term stimulant is
used to refer to therapeutic agents (e.g. natural substances
such as herbs, oils, or essences, or conventional pharmaceutical drugs)
that promote (increase) the activity of a system of the body, or a function
of the body.
For example, caffeine (as in standard, as opposed to de-caffeinated,
coffee and other soft drinks) is a stimulant of the central nervous
Tip: When studying agents that are stimulants, be sure to memorise the
body system/function that they act on.
A more general way to define the term stimulant is
to describe it as the property of therapeutic agents that quicken specific
psysiological function/s of the body.