Plain English Meaning
A code letter at the start of the recorded airport information that tells controllers which version of the recording you heard. Each time the recording is updated, the letter advances to the next one (Alpha, then Bravo, then Charlie, and so on).
Full Definition
A single letter of the phonetic alphabet (Alpha, Bravo, Charlie, and so on) attached to each new ATIS broadcast at a tower-controlled airport. The letter identifies which version of the recorded airport information the pilot has listened to. When conditions or information change, the airport issues a new ATIS recording with the next letter in sequence.
Context Anchor
You encounter an ATIS code before contacting ground, tower, approach, or departure control at an airport that has an ATIS broadcast.