flag: n. A variable or quantity that can take on one of two values;

:flag: n. A variable or quantity that can take on one of two
values; a bit, particularly one that is used to indicate one of two
outcomes or is used to control which of two things is to be done.
"This flag controls whether to clear the screen before printing
the message." "The program status word contains several flag
bits." Used of humans analogously to {bit}. See also
{hidden flag}, {mode bit}.
-- The AI Hackers Dictionary

Related:

• flag n. [very common] A variable or quantity that can take on one of two values;
  a bit, particularly one that is used to indicate...
• mode bit: n. A {flag}, usually in hardware, that selects between two (usually quite different) modes of operation.
  The connotations are different from {flag} bit in...
• bit n. [from the mainstream meaning and `Binary digIT'] 1.
  [techspeak] The unit of information; the amount of...
• trit: /trit/ [by analogy with `bit'] n. One base-3 digit;
  the amount of information conveyed by a selection...
• flag day: n. A software change that is neither forward-
  nor backward-compatible, and which is costly to...
• mode bit n. [common] A flag, usually in hardware,
  that selects between two (usually quite different)...
• bit bashing: n. (alt. `bit diddling' or {bit twiddling}) Term used to describe any of several kinds of low-level programming characterized by manipulation of {bit},
  {flag}, {nybble}, and other smaller-than-character...
• trit /trit/ n. [by analogy with `bit'] One base-3 digit;
  the amount of information conveyed by a selection among...
• bit bashing n. (alt. `bit diddling' or bit twiddling) Term used to describe any of several kinds of low-
  programming characterized by manipulation of bit, flag...