Activation

     (procedure and function) The commencement of execution of a procedure.
     Each call to a procedure is referred to as an activation of that
     procedure.  If the procedure in turn calls another procedure (or calls
     itself recursively), a new activation is said to have occurred.  The
     earlier activation is then referred to as a pending activation.

     Pending activations are often referred to as higher-level activations or
     higher-level procedures.  When one procedure calls another (i.e.,
     creates a new activation), the latter is often referred to as a lower-
     level activation or a lower-level procedure.