1. Literally, a sudden 180-degree turn made by a vehicle, such that it is quickly facing or moving in the opposite direction. Once I realized I was going the wrong way, I had to do a U-turn at the next light.

2. By extension, any sudden and drastic reversal, as of opinion, policy, intention, fortune, trend, etc. After a huge public outcry about the controversial program, the president quickly made a U-turn and announced that he would not support the plan. The disastrous unveiling of the tech giant's latest gadget marks a major U-turn in their otherwise unyielding success.