Look at the manual. It’s typeset as Ti\textit{k}Z …
You may create a new command if you would like to use it more than one time. If you use xspace, it takes care of the following space.
\documentclass{article}
\usepackage{xspace}
\newcommand{\TikZ}{Ti\textit{k}Z\xspace}
\begin{document}
\TikZ with a following space and without \TikZ.
\end{document}
For further reading: Drawbacks of xspace
As said in the the comments, there are some other possible definitions:
Ti\emph{k}Z, which will have an italic k in upright context and an upright k in italic context. So the k is always emphasizes, which may satisfies the source of the name “TikZ ist kein Zeichenprogramm” (TikZ is no drawing software), where the word kein should be emphasized.
\textup{Ti\textit{k}Z} will produce the version with upright T, i and Z and italic k in all cases. This may be preferable, when the original appearance should be kept for the whole document.
\def\tikzname{Ti\emph{k}Z}possible\newcommand*\Tikz{Ti\textit kZ}– Alain Matthes Jul 04 '12 at 16:59\textitto be sure that it’s italic – Tobi Jul 04 '12 at 17:01kis italic, you might want to ensure that theT,i, andZare upright, as in\newcommand*\TikZ{\textup{Ti\textit kZ}}. – Niel de Beaudrap Dec 05 '13 at 01:44