Ghost (also known as ghosts or pig) is a written or spoken word game in which players take turns to extend the letters of a word without completing a valid word.

Ghost can be played by two or more players and requires no equipment, although it can be played with pencil and paper instead of being spoken aloud.

Players take turns to call a letter, adding those letters to a shared, growing word fragment. (For example, if the first player calls "T", the second might call "R" to make "TR".)

Each fragment must be the beginning of an actual word.

The player whose turn it is may — instead of adding a letter — challenge the previous player to prove that the current fragment is actually the beginning of a word. If the challenged player can name such a word, the challenger loses the round; otherwise the challenged player loses the round. If a player bluffs, or completes a word without other players noticing, then play continues.

If a complete word is formed in this way, the player who called the final letter of it loses the round. (Usually some minimum is set on the length of a word that counts, such as three or four letters.) The losing player earns a "letter" (as in the basketball game H-O-R-S-E), with players being eliminated when they have been given all five letters of the word "ghost".

When a round ends, play generally passes to the left.

Since the game tree of Ghost can be derived from the list of combinations of letters that are considered to be words, the game (as played by two players) can be easily "solved" to find a winning strategy for one player.