Comb filter
What you get when a signal is mixed with a delayed copy of itself: a row of evenly spaced notches in the frequency response, like the teeth of a comb.
With a 1 ms delay, the copy lands out of phase at 500 Hz, 1,500 Hz, 2,500 Hz and so on, and those frequencies cancel. It happens by accident all the time: two microphones at different distances from one source, a reflection off a desk, a plugin whose latency isn't compensated. The result sounds hollow and tinny, and no EQ can fix it — the notches are too many and too narrow.
On purpose, it is a building block. A flanger is a comb filter with a sweeping delay. Manfred Schroeder's 1962 reverberator built reverb from parallel feedback comb filters, and the Karplus–Strong algorithm of 1983 uses the same trick to make a burst of noise sound like a plucked string.