Entry 1 sense Webster, 1913 Trilateral /trīl-ăt'-ẽr-əl/ · Tri·lat·er·al · IPA /ˌtrɪˈlæ.təɹ.əl/ 01 a. Having three sides; being three-sided; as, a trilateral triangle. 1. Having three sides; being three-sided; as, a trilateral triangle.(Geom.)