definition environment (lowercase; i'm not familiar with
Definition) is one of three theorem-class environments (styles) defined by
amsthm.sty. you should be able to find the parameters there, or
texdoc amsthmto look at the user's guide.
amsthm also has a
\newtheoremstyle facility, with which you can "roll your own".
this is also described in the
amsthm user's guide.
to view the documented source (in
amsclass.dtx). the "human readable" version is
amsclass.pdf, which can be accessed by