What is a “game term”?

These two questions use the phrase "game term":

  • Is “a special melee attack” an actual game term?
  • What does ‘ally’ mean, as a game term?

Both answers seem to make a distinction between game terms and regular English, but what exactly does it mean for a word or phrase to be a "game term"?

Note, there are many other examples of Q&As that make this distinction, see these search results for more.