Consisting of four; arranged or grouped in fours.[capitalized] In geology, noting that part of the geological series which is more recent than the Tertiary; Post-tertiary. (See Tertiary.)In old chemistry, noting those compounds which contained four elements, as fibrin, gelatin, etc.In mathematics, containing, as a quantic, or homogeneous integral function, four variables.n. A group of four things.Fourfold or tetragonal: said of the symmetry of crystals. See symmetry, 6.