polyiamond
/ˌpɒlɪˈʌɪəmənd/Geometry
noun
A planar shape formed by joining a number of identical equilateral triangles by their edges.
Origin
1960s; earliest use found in Scientific American. From poly- + iamond, by analogy, taking the d- of diamond (by an intentional false etymology) to represent either di- or duo-: compare polyabolo, polyomino, etc.