user19413: I have seen many tutorials on programming an Arduino to control a HC-05 bluetooth module. In that I did not understand the term called 'flag'. What does that mean?

user7297: 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.


19413: <p>I have seen many tutorials on programming an Arduino to control a HC-05 bluetooth module. In that I did not understand the term called 'flag'. What does that mean?</p>
 7297: <p>I've always used flag to indicate a boolean logic value. I would also suggest this is an archaic term.</p>

<p>Was going to come up with my own explanation but found the following at <a href="https://www.quora.com/In-C-language-what-does-flag-mean-Is-it-an-arbitrary-variable-name-Can-I-use-flag-2-or-b-etc" rel="nofollow">In C language, what does flag mean?</a>:</p>

<blockquote>
  <p>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.</p>
</blockquote>
