There exists a shortcut or command which allows one to go straight to the definition of symbol or lemma. (Like "ALT+," in GTAGS for Emacs)
It is quite handy contrary (just imho) to the Coq's one, where one have to do Search command in an IDE.
I forget which combination of keys should I press to look up the definition in standard Isabelle IDE based on jEdit.
Could you please remind me?