"triangular pyramid, solid figure contained by four triangular surfaces," 1560s, from Late Greek tetraedron, noun use of neuter of tetraedros (adj.) "four-sided," from tetra- "four" (see tetra-) + hedra "seat, base, chair, face of a geometric solid," from PIE root *sed- (1) "to sit" (see sedentary). Related: Tetrahedral.