This tag is for questions about the board game Mastermind.
Mastermind is a code-breaking 2-player game created in 1970. It is based on a game called Moo used on the Cambrige Titan computer to practice code-breaking; this game itself was actually based on an older game called "Bulls and Cows". Dr John Billingsley had the idea to create the game, and it was then designed by Mordecai Meirowitz, an Israeli postmaster and telecommunications expert.
One player selects a sequence of coloured pegs and the other player tries to work out what this sequence is, by making guesses and receiving information on how accurate the guesses are, and using deductions to reduce the number of possibilities.
The basic variant is with 6 colours and 4 holes, and repetition is not allowed. The answerer uses white pegs for correct colour in the correct position, and black pegs for correct colour in an incorrect position.
The Wikipedia article on Mastermind can be found here.