I thought it meant the 'roundness' - ie: If it had hit a bump/pothole the wheel would have gone slightly oval but not no more than 0.3mm? It is a measure of how 'out of round' a wheel is
EDIT:
It is, the side to side wobble, like a buckled bike wheel is lateral runout
http://books.google.co.uk/books?id=m...sult#PPA299,M1