\newcommand

LaTeX2e help 1.6

### 2.3.1: \newcommand

       \newcommand{cmd}[args]{definition}
       \newcommand{cmd}[args][default]{definition}
       \renewcommand{cmd}[args]{definition}
       \renewcommand{cmd}[args][default]{definition}

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` | If this optional parameter is present, it means that the command's first argument is optional. The default value of the optional argument is `def`.
`definition` | 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.