read-char

read-char is a built-in function in `C source code'.

(read-char &optional PROMPT INHERIT-INPUT-METHOD SECONDS)

Read a character from the command input (keyboard or macro).
It is returned as a number.
If the character has modifiers, they are resolved and reflected to the
character code if possible (e.g. C-SPC -> 0).

If the user generates an event which is not a character (i.e. a mouse
click or function key event), `read-char' signals an error. As an
exception, switch-frame events are put off until non-character events
can be read.
If you want to read non-character events, or ignore them, call
`read-event' or `read-char-exclusive' instead.

If the optional argument PROMPT is non-nil, display that as a prompt.
If the optional argument INHERIT-INPUT-METHOD is non-nil and some
input method is turned on in the current buffer, that input method
is used for reading a character.
If the optional argument SECONDS is non-nil, it should be a number
specifying the maximum number of seconds to wait for input. If no
input arrives in that time, return nil. SECONDS may be a
floating-point value.