1. past tense and past participle of signal; communicated or indicated something through gestures, sounds, or other means
Usage: British spelling; American spelling is ‘signaled’
2. marked the beginning or occurrence of something; served as a sign of
Usage: British spelling

Examples

• The referee signalled for a timeout.
• She signalled to her friend across the room.
• The train conductor signalled the departure.
• Dark clouds signalled an approaching storm.
• The bell signalled the end of class.
• He signalled his agreement with a nod.