A solid shape with fourteen flat faces. It is a word from geometry, and you will meet it in maths class, in science writing about crystals and foam, and almost nowhere else. Nobody says it in ordinary talk. Note what the word counts: the faces, not the corners or the edges. There is more than one shape with fourteen faces, so the word names a whole group, not one particular thing. The most famous member is the truncated octahedron — take an eight-sided shape and cut off each of its six corners, and you get eight six-sided faces plus six square ones, fourteen in all. Scientists like this one because copies of it stack together with no gaps left over, which is why it comes up when people study bubbles in foam or grains inside metal. You may also see it written 'tetradecahedron', without the 'kai'. Both are correct; the longer one is closer to the Greek. The plural is usually 'tetrakaidecahedra', though 'tetrakaidecahedrons' is not wrong.