Stack-to-pot ratio (SPR) is a metric first published by Matt Flynn, Sunny Mehta, and Ed Miller in Professional No-limit Hold 'em: Volume One. It is calculated by dividing the effective stack against a given opponent by the size of the flop pot. (SPR always refers to the size of the pot and stacks at the flop, i.e. between preflop and flop betting.)
TODO: example etc.