Theora is a free and open lossy video compression format from the Xiph.Org Foundation. It can be used to distribute film and video online (as well as audio and metadata, including subtitles and closed captioning) and on disc without the licensing and royalty fees or vendor lock-in associated with other formats. Xiph.Org Foundation is a non-profit organization that produces free multimedia formats and software tools. Theora is named after Theora Jones, a character in the Max Headroom television program.

Theora scales from postage stamp to HD resolution and is considered particularly competitive at low bitrates. It is in the same class as MPEG-4/DiVX, and like the Vorbis audio codec it has room for improvement as encoder technology develops. Theora has been in full public release since November 3, 2008. The bitstream format for Theora was frozen in 2004, and all bitstreams encoded since that date remain compatible with future releases.

Theora is a variable bitrate DCT (discrete cosine transform)-based video compression scheme. Like most common video codecs, Theora also uses chroma subsampling and block-based motion compensation. Pixels are grouped into various structures, namely super-blocks, blocks and macroblocks. Theora supports intra-coded frames and forward-predictive frames, but not the bi-predictive frames which are found in H.264 and VC-1. Theora also does not support interlacing.

The Theora video-compression format is essentially compatible with the VP3 video-compression format, consisting of a backward-compatible superset. Theora is a superset of VP3, and VP3 streams can be converted into Theora streams without recompression but not vice versa. VP3 video compression can be decoded using Theora implementations, but Theora video compression usually cannot be decoded using VP3 implementations. The libtheora reference implementation provides the standard encoder and decoder under a BSD license.

Theora video streams can be stored in any suitable container format, allowing convenient file-sharing of source material for such uses as archiving and transcription of original video programming. Most commonly it is found in the Ogg container with Vorbis or FLAC audio streams which provide a completely open, royalty-free multimedia format. It can also be used with the Matroska container.

Theora is derived from the proprietary VP3 codec, released into the public domain by On2 Technologies. It is comparable in design and bitrate efficiency to MPEG-4 Part 2, early versions of Windows Media Video and Real Video.

Theora is established as a video format in open-source applications and is the format used for Wikipedia’s video content. Theora is supported by browsers such as Mozilla Firefox 3.5 and later versions, Google Chrome, Tizen, SeaMonkey, Konqueror, Opera and Midori. Theora is also supported by DirectShow, Gstreamer, Phonon, QuickTime, Silverlight, FFmpeg, Helix Player, Miro Media Player, MPlayer, VLC, xine and Dragon player.