An electronic system that warns you when there are other cars or motorcycles in your blind spot.
The warning typically comes via flashing lights in the side mirrors. Some systems also add a loud audible alert if you activate the turn signal to indicate that you're planning to turn in the direction of a detected vehicle.
Honda uses a slightly different approach to blind spots with its Lane Watch system. On cars equipped with Lane Watch, signaling to the right activates a camera which then projects video of the right side blind spot area onto a screen on the dashboard.
All of these systems can be turned off via a button on the dashboard or turn signal stalk.