An abbreviation commonly used among anime viewers to refer to the "ending" or "credits" of an anime show. Often times it contains a short animation along with a song. It has also been used in the context of referring just to the song portion of the ending.
Person A: "What did you think of the show's ED?"
Person B: "Which season ending are you talking about? 1 or 2?"
Person A: "Season 2"
Person B: "Ah, well the animation was mediocre, but the music was put together nicely."