Use the amsthm package by the American Mathematical Society.
Standard LaTeX only offers a \newtheorem command that will create numbered environments. I used this package solely for its \newtheorem* command that allows the creation of unnumbered environments.
In the preamble:
\usepackage{amsthm} \newtheorem*{definition}{Definition}
In the document:
\begin{definition}\label{def:data-parallelism} Data-parallel programming is a form of parallel programming in which the programmer specifies the distribution of data over the processors. It is left to the compiler to choose and implement the distribution of the actual computations over the processors. \end{definition}