Advanced Audio Coding (AAC), also known as MPEG-2 Part 7, and also MPEG-4 Part 3 in a slightly modified form, is a digital audio encoding and lossy compression format. It was popularized by Apple Computer through its iPod and iTunes Music Store.
AAC was designed as an improved-performance codec relative to MP3 (which was specified in MPEG-1) and MPEG-2 Part 3 (which is also known as "MPEG-2 Audio" or ISO/IEC 13818-3).
AAC was promoted as the successor to MP3 for audio coding at medium to high bit rates.
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wikipedia's full definition. In the future, I suggest using google or
www.wikipedia.org to get definitions. This forum is more for how's and not whats.