transient-mark-mode

transient-mark-mode is a variable defined in `C source code'.
Its value is

nil

Documentation:
Non-nil if Transient Mark mode is enabled.
See the command `transient-mark-mode' for a description of this minor mode.

Non-nil also enables highlighting of the region whenever the mark is active.
The variable `highlight-nonselected-windows' controls whether to highlight
all windows or just the selected window.

Lisp programs may give this variable certain special values:

- A value of `lambda' enables Transient Mark mode temporarily.
It is disabled again after any subsequent action that would
normally deactivate the mark (e.g. buffer modification).

- A value of (only . OLDVAL) enables Transient Mark mode
temporarily. After any subsequent point motion command that is
not shift-translated, or any other action that would normally
deactivate the mark (e.g. buffer modification), the value of
`transient-mark-mode' is set to OLDVAL.

You can customize this variable.