Turing machine
For decidability theory a Turing machine is said to decide a language if it is always able to determine whether a given word is contained in a certain language or not. Therefore, the machine usually has two special states marked as Accept and Reject. After a while one of the two states will be reached (depending on the input word) and the machine is halted. If only one of the two states will ever be reached, the Turing machine is said to semi-decide a language.