1. (Mathematics) a solid figure having 20 faces. The faces of a regular icosahedron are equilateral triangles. [C16: from Greek eikosaedron, from eikosi twenty + -edron-hedron]. (aɪˌkoʊ səˈhi drən, aɪˌkɒs ə-). n., pl. -drons, -dra (-drə).