Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication. The algorithms are openly documented and may be used free of software patent restrictions. Development of the format was maintained by the Xiph.Org Foundation (as part of the Ogg codec family) and later coordinated by the Opus working group of the Internet Engineering Task Force (IETF).
CELT was meant to bridge the gap between Vorbis and Speex for applications where both high quality audio and low delay are desired. It is suitable for both speech and music. It borrows ideas from the CELP algorithm, but avoids some of its limitations by operating in the frequency domain exclusively.
The original stand-alone CELT has been merged into Opus. Therefore CELT as stand-alone format is now abandoned and obsolete. Development is going on only for its hybridised form as a layer of Opus, integrated with SILK. This article covers the historic, stand-alone format, for the integrated form and its evolution since the integration into Opus see the article on Opus.