# Numbered conditions in definition

How can I make numbered conditions in definitions, for example and ideal I is called... if the following two conditions holds.

i) A

ii) B

I tried to use enumerate and \item but it just looks bad.

You could load the enumitem package and set up the enumerate environment inside the defn environment to use lowercase-roman "numbers" followed by a ). In any cross-references to these items, the trailing parenthesis should probably be omitted.

\documentclass{article}
\usepackage{enumitem}
\usepackage{ntheorem}
\theoremstyle{plain}
\theorembodyfont{\upshape}
\newtheorem{defn}{Definition}

\begin{document}
\begin{defn} \label{defn:three}
Consider a system that satisfies the following conditions:
\begin{enumerate}[label=\roman*), ref=\roman*]
\item \label{item:mumble} Mumble \dots
\item Grumble \dots
\item \label{item:cough} Cough cough \dots
\end{enumerate}
Such a system is called a \dots
\end{defn}

Conditions \ref{item:mumble} and \ref{item:cough} in Definition \ref{defn:three} amount to \dots
\end{document}