The word Propagation refers: to multiply via natural reproduction, to reproduce, or to breed. We use the term "frag" to descibe the action of propagation. "Fragments" or "Frags" are taken from the original parent colony, hence reproducing the original coral into identical smaller portions. We often dont have the means to let coral reproduce naturally in our tanks, so propagtion is a means to devide corals, and sustain our hobby. (Note, not all corals can be "propagated" as such, and methods vary).