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 as CELT while not exceeding CELT performance for music.
See e.g. a comparison done under fall 2012: http://www.hydrogenaudio.org/forums/index.php?showtopic=97913&st=0 (for speech Opus performs better, due to SILK).
For archiving of music, especially for possible playback on resource-constrained devices CELT has thus a value on its own, not necessarily as a part of Opus.
It is a loss that standardization of CELT on itself did not complete (https://tools.ietf.org/html/draft-valin-celt-codec-02).