noun

• (nautical) A flag flown by a ship to show the presence on board of the admiral; the admiral himself, or his flagship.
• (nautical, often used attributively) A signal flag.
• (computer science) A variable or memory location that stores a Boolean true-or-false, yes-or-no value, typically either recording the fact that a certain event has occurred or requesting that a certain optional action take place.
• (computer science) In a command line interface, a command parameter requesting optional behavior or otherwise modifying the action of the command being invoked.
  "This will be used as a help message if the user passes in the --help flag, like so: […]"

verb

• (often with down) To signal to, especially to stop a passing vehicle etc.
  "Please flag down a taxi for me."
• (often with up) To note, mark or point out for attention.
  "I've flagged up the need for further investigation into this."
• (computing) To signal (an event).
  "The compiler flagged three errors."
• (computing) To set a program variable to true.
  "Flag the debug option before running the program."

verb

• (intransitive) To weaken, become feeble.
  "His strength flagged toward the end of the race."

noun

• (obsolete except in dialects) A slice of turf; a sod.
• (geology) Any hard, evenly stratified sandstone, which splits into layers suitable for flagstones.