AAC
Also known as: Advanced Audio Coding, .m4a, .aac, aac file
AAC (Advanced Audio Coding) is a lossy audio format designed as the successor to MP3, delivering better sound at the same file size. It is the default for Apple Music, iTunes, and YouTube, and is what iPhones use for recorded audio.
- Lossy successor to MP3 with better quality per byte
- Default for Apple Music, iTunes, and YouTube
- Often stored inside an .m4a container on iPhone
Why AAC replaced MP3
AAC uses more efficient lossy compression than MP3, so at a given file size it generally preserves more detail. At a given quality target, it produces a slightly smaller file. That efficiency is why it became the standard for Apple Music, iTunes purchases, and most streaming.
On iPhone, AAC audio is often stored inside an .m4a file — another case where the container (M4A) and the codec (AAC) are different things. Voice Memos and many video soundtracks use AAC too.
Still lossy
Like MP3, AAC throws away data to save space, so it is not a master-quality format — for that you need a lossless codec such as FLAC or Apple Lossless. For everyday listening on phones and earbuds the difference is rarely audible.
AAC files are compact, so an audio library rarely dominates storage. Photos, and especially video, almost always take far more room.