节流阀，俗称油门，或称气门、气阀，是一个可以调节液体压力的构造，可调整进入引擎的空气量，进而调整引擎的出力。

节流阀可调整空气进入汽油引擎的量。 在现代汽油引擎中，节流阀通常控制汽油与空气的雾状混合物进入气缸的量。

而由于柴油引擎是使用压力来引燃，所以并不需要节流阀，而是使用蝶阀（butterfly valve）来调整油量。

A throttle is the 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 the use of a throttle), but usually decreased. 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 is often termed a throttle (in an aviation context) is more correctly called a thrust lever, particularly for jet engine powered aircraft. For a steam engine, the steam valve that sets the engine speed/power is often known as a regulator.