Triangulum Syllabification: (Tri·an·gu·lum)Pronunciation: /trīˈaNGgyələm/ AstronomyDefinition of Triangulum a small northern constellation (the Triangle), between Andromeda and Aries. (as genitive Trianguli /trīˈaNGgyəlē, -ˌlī/) used with a preceding letter or numeral to designate a star in this constellation:the star Beta TrianguliOrigin:LatinTriangulum in other Oxford dictionaries Definition of Triangulum in the British & World English dictionary