this answer presupposes that you are using the
amsthm package, and not one of the ams document classes, which incorporate the
amsthm facilities, but sometimes with slightly different assumptions.
you can adapt the spacing around the
definition environment using this redefinition.
\thm@preskip <dimen> \relax
\thm@postskip <dimen> \relax
(it isn't necessary to redefine the end of the environment; it's set up this way since often the pre- and post-skips are equated by the statement
\thm@postskip\thm@preskip, and that is most often done at the same time the preskip is set so that the relation between the settings is more easily recognized.