ORIGINAL: mulligan
Wow, had to check in on this thead, having so many replies for such a simple topic. Tip stall is simply the outer portion of the wing stalling before the inboard portion, which causes you to lose roll control (because your ailerons are in the stalled portion of the wing). Washout or a different root vs. tip airfoil section is used to ensure the inboard portion of the wing stalls first- wing stalls, you still have aileron (roll) control, the airplane naturally pitches down a little, the wing starts flying again- that's it.
Yes, but the real meat of the dicussion is: how often is this a really accurate description of an asymetric (rolling) stall? Not very often. Most of the time people say "tip stall" that's not what happened. You don't need a tip stall to have an accelerated rolling stall (snap/spin entry).