Entry 1 sense Webster, 1913 batholith /(băth"ō̇*lĭth)/ · bath·o·lith · IPA /bˈæθəlɪθ/ 01 n. A large mass of intrusive igneous rock believed to have solidified deep within the earth. 1. A large mass of intrusive igneous rock believed to have solidified deep within the earth. Syn. batholite, pluton, plutonic rock.