The code what I've got:
\documentclass[a4paper]{article}
\usepackage{amsthm}
\begin{document}
\textbf{Definition 1.1} The definition contains following contents:
\begin{enumerate}
\item The first item
\item The second item
\item This item may long enough to take up more than a line like this sentence. Line Breaking \ldots
\end{enumerate}
\end{document}
Result of this code:

The result what I want:

The things have to be changed: space between lines, item format.
:)Using theenumitempackage, you can customize theenumerateenvironment to look like that, say, with\setlist[enumerate]{label*=(\arabic*),itemsep=0ex,parsep=0ex}. IMHO, since this change is global and affects allenumerateenvironments you might have throughout the whole document, it would be wiser to create a new list based onenumerate(theenumitempackage also offers you this feature, via\newlist). – Paulo Cereda Feb 06 '14 at 07:12