The (supposed) unified form of the electroweak force and the strong nuclear force. The electrostrong transition is hypothesized to take place somewhere near the energy level of 100 TeV, which is well beyond today's particle accelerators. This unified force is predicted by various attempts to create a Grand Unified Theory that have, to date, had little success.
I suppose I should have made it more clear that the electrostrong force is a name for a hypothetical theory that doesn't exist, and worse yet I have none of the background to explain why there most likely won't be a stepping-stone theory that unifies electromagnetic, weak, and strong but not gravity.