OS General Concepts Question:

It is a useful, memory-saving technique for multiprogrammed
timesharing systems. A Reentrant Procedure is one in which
multiple users can share a single copy of a program during
the same period. Reentrancy has 2 key aspects: The program
code cannot modify itself, and the local data for each user
process must be stored separately. Thus, the permanent part
is the code, and the temporary part is the pointer back to
the calling program and local variables used by that
program. Each execution instance is called activation. It
executes the code in the permanent part, but has its own
copy of local variables/parameters. The temporary part
associated with each activation is the activation record.
Generally, the activation record is kept on the stack.