signal

A picture, or a light, or a movement of a hand, or other thing that tells people to do something.

Example: When I make this signal with my hand, everybody walk to the front of the room.

A picture, or a light, or a movement of a hand, or something else that lets someone recognize another person, a ship, an airplane or something else.

Example: The sailor saw the signal on the other ship and so recognized it was the enemy.

When someone or something signals to someone, they use a signal to tell the person something.

Example: The red light signals to the cars to stop.