\newcommand

\newcommand{cmd}[args]{def} \renewcommand{cmd}[args]{def}

These commands define (or redefine) a command.

cmd A command name beginning with a \. For \newcommand it must not be already defined and must not begin with \end; for \renewcommand it must already be defined.
args An integer from 1 to 9 denoting the number of arguments of the command being defined. The default is for the command to have no arguments.
def The text to be substituted for every occurrence of cmd; a parameter of the form #n in cmd is replaced by the text of the nth argument when this substitution takes place.