In music theory, a confused and complex subject. In normal use, it stands for Key Signature as in 'this tune is in the key of G' or more commonly 'this tune is in G'. In a session situation, a kindly player might call out the key (signature) in a kind of Phonetic Alphabet:
Sometimes with minor appended - Apple minor for Am. It is more accurately the Mode since a modal tune will have a different Tonic or starting note than a simple major or minor tune. For example, G major and D Mixolydian would both appear to be in the key of G from the key signature.