The term 'TWICE TETRA-' can be completed with the crossword solutions OCTO and OCTA. OCTO is a four-letter word meaning 'EIGHT' and OCTA is also a four-letter word meaning 'EIGHT'. Together, these two words form the prefix 'TWICE TETRA-', which means 'sixteen'. This prefix is used to describe a quantity that is twice the number of sides on a tetrahedron, which is a three-dimensional shape with four triangular faces.