There are several ways one may format a definition in latex, but each has their problems.
Use the amsthm package, and the usual style for theorems. This will result in everything italicized. It is difficult to catch the term you are defining, even if you non-italicize it.
Use the amsthm package, and the style for definitions. This time the term you are defining is the only word/phrase italicized, but the problem is that one does not know where the definition ends. Unlike the proof environment, there is no QED marker, so it is unclear where the definition ends and when the next paragraph starts.
End a definition with a QED-type marker (like a flower or whatever). The problem with this is that there is an over-abundance of markers. Flowers and square boxers.
How do you format your definitions in latex?