(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. The show experienced a strong revival in the 1970s when it became syndicated across America, including on local "UHF" channels in addition to being shown on major networks. In syndication, Star Trek was often aired late at night (after 9 or 10PM) or on weekends. Due to the different time allotments for syndicated programs, nearly all Star Trek episodes were cut of certain scenes to allow for running in their syndicated forms. This led to a popularity in "uncut" episodes which were available on VHS tapes (and later DVDs) or shown in their entirety as part of special programing, such as the 20th anniversary of Star Trek in 1987.
Syndicated Star Trek programing continued well into the 1990s and early 2000s; however, some later showings presented the episodes in their entire form, without cuts, or cut the episodes in different ways. This was most evident on the Sci-Fi Channel, which began showing its Sci-Fi Channel Star Trek Special Edition in the late 1990s, which either cut additional scenes from those in syndication or showed the episodes in their entire form.
With the rise of digital streaming (see below), syndicated Star Trek episodes, saved on such media formats as VHS, have become a rarity and are even sought out for their uniqueness as collector's items.
The producers of Star Trek: The Next Generation originally sought a lower-pressure environment than syndication, when Paramount Television CEO John S. Pike approached the three national networks in his 1986 search for an outlet for the new show, but failed to engender network interest for it, including the at the time by Brandon Tartikoff headed NBC, which had originally aired Star Trek: The Original Series and Star Trek: The Animated Series. (William Shatner Presents: Chaos on the Bridge). Tartikoff incidentally, made good on his error in judgement later on when he, now as Paramount head, became responsible for the inception of both the Next Generation Star Trek films and Deep Space Nine, which he, somewhat ironically, ordained to premiere in syndication as well. (Star Trek: The Next Generation - The Continuing Mission, pp. 154-157)
When the owner of Star Trek, Paramount Pictures, launched its own network, UPN in 1995, Star Trek: Voyager was the flagship series. UPN was intended as a de facto fifth national broadcaster, after NBC, ABC, CBS and the relative newcomer Fox Television, which was founded around the time when Pike was making his rounds for The Next Generation.
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 Friday night instead, where its ratings fell, leading to cancellation. In 2007, Sci-Fi acquired the first-time rights to re-air Enterprise in syndication, starting on 8 January 2007 alongside its own series many felt was in large part responsible for its demise, Ronald D. Moore's Battlestar Galactica.
From the mid-2000s onward, classic syndication rapidly started to loose relevance as an increasingly outdated format with the industry-changing advent of the digital streaming services such as Netflix, Prime Video or Hulu, almost all of them streaming one or more Star Trek productions, and which the traditional networks also started to use – if not establishing their own streaming service for that matter, as had been the case with CBS All Access for example. As such, these services can be considered the successor, replacement, or modernized version, of syndication. Because of the nation-wide coverage of such services, still existing local television stations rarely buy, if at all, Star Trek broadcast rights anymore, as do foreign television stations for that matter, for pretty much the same reasons.