If use an (i.e., any) environment for your definitions, there will be a little vskip after it, so it is easy to see where it ends.

You can control the font used in the definition (or any) environment. Use \tt or \sf if you want different than \it.

Put the \emph{defined} word in \emph as here. That will automatically use an alternate font: if the text is otherwise \rm, it will use \it and if the text is in \it, it will use \rm. I think that under some documentclass-es it might produce \bf, but that is usually by the choice of the publisher.