A throttle is a mechanism by which fluid flow is managed by constriction or obstruction. An engine’s power can be increased or decreased by the restriction of inlet gases (i.e., by using a throttle), but it usually falls.
The term throttle has come to refer, informally and incorrectly, to any mechanism by which the power or speed of an engine is regulated. What has often been termed a throttle (in an aviation context) is more correctly called a thrust lever.
The steam valve that sets the engine speed/power is often known as a regulator for a steam engine.