What is a flag in programming Arduino?
I've always used flag to indicate a boolean logic value. I would also suggest this is an archaic term.

Was going to come up with my own explanation but found the following at In C language, what does flag mean?:


  A 'flag' is a variable that is supposed to signalize something. Think
  about the word itself - flags are used in sending signals. In our
  world of programming a basic boolean flag can either be "up" or
  "down", either true or false.