(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.
The original Star Trek was aired on NBC, 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, 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.