Washout is a twist in the wing where the angle of attack at the wing tip is lower than the angle of attack at the root of the wing. This causes the root portion of the wing to stall before the tip usually resulting in a controlled stall (ie: straight ahead)

If the tip stalls first it usually results in a wing dropping with a resultant spin or snap roll.

That's the easy definition, I'm sure somebody will come along and give you the aerodynamics later...but it's too late for me. ;-)

Don