The Free On-line Dictionary of Computing (30 December 2018):

cooperative multitasking

    A form of multitasking where it
   is the responsibility of the currently running task to give up
   the processor to allow other tasks to run.  This contrasts
   with pre-emptive multitasking where the task scheduler
   periodically suspends the running task and restarts another.

   Cooperative multitasking requires the programmer to place
   calls at suitable points in his code to allow his task to be
   descheduled which is not always easy if there is no obvious
   top-level main loop or some routines run for a long time.
   If a task does not allow itself to be descheduled all other
   tasks on the system will appear to "freeze" and will not
   respond to user action.

   The advantage of cooperative multitasking is that the
   programmer knows where the program will be descheduled and can
   make sure that this will not cause unwanted interaction with
   other processes.  Under pre-emptive multitasking, the
   scheduler must ensure that sufficient state for each process
   is saved and restored that they will not interfere.  Thus
   cooperative multitasking can have lower overheads than
   pre-emptive multitasking because of the greater control it
   offers over when a task may be descheduled.

   Cooperative multitasking is used in RISC OS, Microsoft
   Windows and Macintosh System 7.

   (1995-03-20)