Constrained-Energy Lapped Transform (CELT) Codec. Experimental low-delay wide-band audio codec from the creator of Speex.
It is deprecated in favor of Opus which CELT became a part of.
Unfortunately Opus contains about twice as much code while being only marginally better for music compared to pure CELT (for speech Opus is preferable due to SILK). For resource-constrained environments CELT might have a value on its own.