All-pass filter
A filter that passes every frequency at unchanged level but shifts its phase – which sounds pointless until you combine it with something else.
On its own an all-pass filter is nearly inaudible, since the ear is poor at hearing phase. Mix it back with the dry signal, though, and the frequencies that ended up out of phase cancel. A chain of four to twelve all-pass stages with their corner frequency swept by an LFO is exactly what a phaser is.
Manfred Schroeder used them in 1962 for the opposite purpose in reverb: an all-pass with a long delay smears one echo into many without colouring the tone, and a few in series turn a sparse signal dense. They also straighten out phase in loudspeaker crossovers, and in biquad form they turn up inside nearly every EQ.