I always thought that the running term "hitting the wall" was fairly equivalent to "bonking" which is used more in the cycling/triathlon world. As onimity says, it's the point of glycogen depletion.

Bonking for me means dizzy, delirious, incoherent, unable to stand straight, chills, etc. It always amazes me how easy it is to not notice that I'm bonking while on the bike. Then, when I stop the bike or get off and walk around I'm suddenly very very woozy.

An active bonk is different from "dead legs" which are often a result of overtraining, sickness, medication, or not properly replenishing glycogen after a previous ride.