overwrite-mode

overwrite-mode is an interactive compiled Lisp function in `simple.el'.

It is bound to <insertchar>, <insert>.

(overwrite-mode &optional ARG)

Toggle Overwrite mode.
With a prefix argument ARG, enable Overwrite mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
the mode if ARG is omitted or nil.

When Overwrite mode is enabled, printing characters typed in
replace existing text on a one-for-one basis, rather than pushing
it to the right. At the end of a line, such characters extend
the line. Before a tab, such characters insert until the tab is
filled in. C-q still inserts characters in
overwrite mode; this is supposed to make it easier to insert
characters when necessary.