flat spot

Short definition from Wiktionary

Noun

1. 1
   A small patch of a vehicle tyre that has been flattened by abrasion with the road surface, so that the tyre is no longer smoothly circular at that point; usually caused by the wheel locking under heavy braking.

Verb

1. 1
   To cause a flat spot on (a vehicle tyre), usually by locking the wheels under heaving braking.