A device that keeps a motor from spinning to quickly. In modern cars, a rev limiter is typically integrated into the engine management software. In that case, the rev limiter operates by retarding timing, or refusing to fire a spark when the engine speed exceeds the rev limiter.

Without a rev limiter, it is possible for an internal combustion engine to spin itself faster and faster until it breaks. Before engine management computers, rev limiters were integrated into the ignition, typically at the coil. However, few production cars came with rev limiters stock; it was typically a race or hot rodder's modification.

The feel of hitting a rev limiter is a bit disconcerting, since the engine has been spinning up and spinning up, then suddenly it just sits at one RPM and makes funny "bup-bup-bup" noises while lurching a little bit. It's harmless in most cars, albeit freaky the first time.

While a rev limiter will prevent the engine from spinning itself up to high RPM's, it can do nothing about mechanical over-revs (when you accidentally shift into second gear rather than fourth gear at 100mph, for example). In that case, the transmission spins the engine way too fast, and spark timing (or presence of spark) has nothing to do with it.