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?
It's the Control key on Windows and Linux systems and the Command key on macOS systems. See Section 1.2 in the Isabelle/jEdit documentation.