flag

ot noun

1. A piece of cloth, often decorated with an emblem, used as a visual signal or symbol.
2. An exact representation of a flag (for example: a digital one used in websites).
3. A flag flown by a ship to show the presence on board of the admiral; the admiral himself, or his flagship.
4. (often used attributively) A signal flag.

fe’l verb

1. (often with down) To signal to, especially to stop a passing vehicle etc. Please flag down a taxi for me.
2. To convey (a message) by means of flag signals. to flag an order to troops or vessels at a distance
3. (often with up) To note, mark or point out for attention. I've flagged up the need for further investigation into this.
4. To signal (an event). The compiler flagged three errors.