The number of drivers, each dedicated to a specific frequency range, in a car speaker: (1-way) single driver; (2-way) woofer and tweeter; (3-way) woofer, midrange, tweeter; (4-way) woofer, midrange, tweeter, and either an additional midrange or a supertweeter.
A 2-way speaker splits the music into highs and lows, even if it uses multiple tweeters. A 3-way speaker covers lows, midrange, and highs, even though it may have two tweeters covering the highs.