A language game is a type of cipher, or relex, of a natural language that involves adapting words from the natlang by following a set of rules that allow each word to undergo predictable changes. The grammar is not changed.
For instance, in Pig Latin, the most widely known and used language game in the English-speaking world, the onset of the first syllable in each English word is moved towards the end, and followed by -ay, so that "dollar" becomes "ollar-day" and "scrounge" becomes "ounge-scray". If the word begins with a vowel sound, the word is simply followed by -ay (so that "elephant" becomes "elephant-ay") in one version, or followed by -way (so that "elephant" becomes "elephant-way") in another version.
Other language games: