tri·lateral I. \“+\adjective Etymology: Latin trilaterus (from tri- + later-, latus side) + English -al — more at lateral 1.: having three physical or material sides < a trilateral figure > 2.: involving three interests often to the disadvantage of other interested parties < a trilateral agreement > < trilateral oligopoly in the market place — G.L.Bach > • tri·laterality\(|)trī+\noun • tri·laterally\(ˈ)trī+\adverb II. noun : a figure having three sides : triangle