\ifundefined

\ifundefined{cs} t \else f \fi expands the t text if the control sequence \cs is undefined or has been \let to \relax, and the f text otherwise.

Since \ifundefined is not a primitive conditional, it cannot be used in places where TeX might skip tokens "at high speed", e.g., within another conditional---TeX can't match up the \if's and \fi's.

This macro was taken directly from The TeXbook, page 308.