(written from a Production point of view)
Syndication is the occurrence of local television stations purchasing individual television shows outside of a network context. Some stations carry network programming at peak hours, such as prime-time and weekends, and air syndicated series at other times. Others thrive solely on syndicated shows. Star Trek: The Next Generation and Star Trek: Deep Space Nine were both syndicated, and actually the first Star Trek shows to be released directly through syndication, instead of premiering on one of the three national broadcasters, constituting an innovative industry novelty at the time when The Next Generation was launched. (William Shatner Presents: Chaos on the Bridge)
The original Star Trek was aired on NBC, one of the three traditional national broadcasters, where it was canceled after only three seasons. This led the producers of TNG to seek a lower-pressure syndicated environment. When the owner of Star Trek, Paramount Pictures, launched its own network, UPN (de facto intended to be the fifth national broadcaster at the time, after NBC, ABC, CBS and relative newcomer Fox Television), in 1995, Star Trek: Voyager was the flagship series. Voyager ended in 2001 and was immediately replaced with Star Trek: Enterprise, which was canceled after only four seasons as UPN attempted to change its image. Like the original Trek, Enterprise was nearly canceled a year earlier than it was, but was spared and moved to the deadly Friday night instead, where it perished.