Entry 1 sense Webster, 1913 Glabellum · Gla·bel·lum · IPA /ɡləˈbɛləm/ 01 n. The median, convex lobe of the head of a trilobite. See Trilobite. pl. Glabella ((#)) 1. The median, convex lobe of the head of a trilobite. See Trilobite.(Zool.) See: Trilobite