Variable: help-char
help-char is a customizable variable defined in keyboard.c.
Value
8
Documentation
Character to recognize as meaning Help.
When it is read, do (eval help-form), and display result if it's a string.
If the value of help-form is nil, this char can be read normally.
Probably introduced at or before Emacs version 19.20.
Source Code
// Defined in /usr/src/emacs/src/keyboard.c
DEFVAR_LISP ("help-char", Vhelp_char,
doc: /* Character to recognize as meaning Help.
When it is read, do `(eval help-form)', and display result if it's a string.
If the value of `help-form' is nil, this char can be read normally. */);