Delay (audio effect)
An effect that plays back a delayed copy of the signal, usually with feedback so the echo repeats and decays.
The principle could hardly be simpler: write audio into a ring buffer, read it back a few hundred milliseconds later and mix it with the original. Feedback sets how many repeats follow, and in a DAW the time is usually locked to tempo so echoes land on eighth notes. Ping-pong delay bounces the repeats between left and right.
Before digital, it was done with tape. The Echoplex of 1961 and Roland's RE-201 Space Echo of 1974 ran a tape loop past several playback heads, and the wow, flutter and wear of that tape are exactly what today's plugins spend their CPU imitating. Make the delay very short and sweep it, and it turns into a comb filter — which is how flangers and choruses work.