A program component that allows structuring of a program in an unusual way. A coroutine resembles a subroutine, with one important difference. A subroutine has a subordinate position relative to the main routine: it is called and then returns. Coroutines, however, have a symmetric relation: each can call the other. Thus a coroutine is resumed at a point immediately following its call of another coroutine; it never returns, but terminates its operation by calling (resuming) another coroutine.
Coroutines are not commonly found in high-level languages. They are particularly useful as a means of modeling concurrent activity in a sequential machine.