1. noun /flæɡ/
b) A flag flown by a ship to show the presence on board of the admiral; the admiral himself, or his flagship.
2. verb /flæɡ/
a) To mark with a flag, especially to indicate the importance of something.
Please flag down a taxi for me.
b) To signal to, especially to stop a passing vehicle etc.
Ive flagged up the need for further investigation into this.
Wikipedia foundation.