Upmixing, in its most general sense, is the opposite of downmixing. This means that upmixing is a process that can take some number of audio channels and turn them into a greater number of audio channels, and in practice this process transforms 2-channels into 5.1 channels. Upmixing is commonly used to better integrate legacy two-channel mono, stereo, or surround encoded content into 5.1 channel programs. Chosen properly, upmixing further speeds the transition to 5.1 by helping out legacy content, and by assisting in the creation of new 5.1 channel material.
In most cases, it should not be confused with synthesizing processes, such as the old mono to stereo units which produced varied results in stereo and mono. Upmixing relies on power steering of some sort, very much like a matrix decoder operates. The primary difference is that unlike a matrix decoder, an upmixer must be able to survive being Dolby Digital (AC-3) encoded, downmixed, and possibly even matrix decoded, and, still must sound good in 5.1. This is a nearly impossible order for a consumer-grade upmixer to fulfill, and is challenging for even the best professional upmixing gear.
So, being the opposite of downmixing might at first have seemed like too simple an answer, but it is a critical aspect of the process since most DTV viewers do so through the stereo or mono speakers built into their sets. This somewhat misunderstood process is extremely common in modern broadcast, and the proof that it works as advertised is the large base of satisfied consumers--mono, stereo, and surround.
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