Crossover network
A filter network that splits an audio signal into frequency bands, so the woofer gets the bass and the tweeter gets the treble.
No single driver covers the whole audible range. A passive crossover lives in the speaker cabinet after the amplifier and is built from inductors, capacitors and resistors. An active one splits the signal before amplification, these days usually in a DSP, and needs an amplifier per band — more hardware, but full control over slope, delay and level.
The hard part is the overlap: two filters crossing each other should sum to a flat response and matching phase. Siegfried Linkwitz and Russ Riley published a filter alignment in 1976 that does exactly that, and Linkwitz–Riley remains the default choice. The same split sits inside every multiband compressor, just without the speakers.