These are two names for the same device, unless I'm missing something. I don't see any reason to have two different tags, especially when we have the capability of making one an alias for the other.
All SCRs are thyristors, but there are many thyristors which are not SCRs (chiefly in current use are triacs (and alternistors) and diacs, but also programmable unijunction transistors), Quadracs (triac-diac combinations) and the occasional GTO (Gate Turn Off, not Gran Turismo Omologato).
Questions involving them are sufficiently infrequent and appear to be answered by a small subset of the usual responders so I don't think the merge (to "thyristor" only) would have any bad effects, other than that a substantial percentage of folks with an SCR question may not give the proper tag to their query if they don't recognize (or think of) the thyristor tag (see below comment about North America vs. the rest of world).
Note: I have serious doubts about the validity of the Wikipedia claim that "SCR" was ever a trade name- no citation is given for that claim. The 1972 SCR manual (neither the online one or the one on my shelf) does not have any such indication, and the fellow (F.W. “Bill” Gutzwiller) who says he coined the term as it was developed from work originally done at Bell Labs says:
"In the international arena the SCR became known as the “thyristor”, probably because this terminology had less American connotation."
I certainly remember Motorola parts in the early 1970s using the "SCR" name, and do not remember it being unique to General Electric.
I suspect the author(s) of the Wikipedia article is/are conflating SCR with triac, which was a originally a trade name (now genericized).
I beg to differ from other people who have answered: I don't think they should be merged.
As already pointed out in other answers/comments there are two school of thoughts: one that sees SCR and Thyristor as synonymous, and another that sees an SCR as a particular type of Thyristor (class which comprises also DIACs, TRIACs, GTOs, PUTs and other devices having an NPNP structure - or behaving as such).
I belong to this latter because I was taught so decades ago, so I can't provide references off the top of my mind, but I dare say this is a common view here in Europe (I'm Italian). I also remember quite clearly an old German databook (circa 1985; ECA was the publisher) named "Thyristoren" (German word for "Thyristors") which listed all the then-known SCRs, TRIACs, DIACs, etc. with their main characteristics and their equivalent parts.
I managed to find a crappy photo online of that book here:
As you may see (with some effort) on the cover there are the symbols of SCRs, TRIACs and other devices.
So the problem with merging those tags is that some people might want to ask a general question about Thyristors (the class of devices) and they wouldn't have a proper tag to use.
Thyristors (sometimes termed SCR's, meaning semiconductor-controlled rectifiers) are one of the oldest (1957 in GE research laboratories) types of solid-state power device and still have teh highest power-handling capability. They have a unique four-layer construction and are a latching switch that can be turned on by the control terminal (gate) but cannot be turned off by the gate
This is the definition captured in "Power Electronics: Converters, Applications and Designs": Mohan/Underland/Robbins: ISBN 978-0471226932, which within certain circles is classed as the bible of power electronics.
The name thyristor is a generic term for a bipolar semiconductor device which comprises four semiconductor layers and operates as a switch having a latched on-state and a stable off-state. Numerous members of the thyristor family exist. The simplest device structurally is the silicon-controlled rectifier (SCR) while the most complicated is the triac.
This is the definition given in Principles and Elements of Power Electronics Prof. B.Williams ISBN: 978-0-9553384-0-3
A thyristor is a solid-state semiconductor device with four layers of alternating P- and N-type materials. It acts exclusively as a bistable switch, conducting when the gate receives a current trigger, and continuing to conduct until the voltage across the device is reversed biased, or until the voltage is removed (by some other means). A three-lead thyristor is designed to control the larger current of the Anode to Cathode path by controlling that current with the smaller current of its other lead, known as its Gate. In contrast, a two-lead thyristor is designed to switch on if the potential difference between its leads is sufficiently large (breakdown voltage).
Some sources define silicon-controlled rectifier (SCR) and thyristor as synonymous. Other sources define thyristors as more ornately constructed devices that incorporate at least four layers of alternating N-type and P-type substrate.
This is the definition from Wikipedia, the well known "accurate" repository of information: https://en.wikipedia.org/wiki/Thyristor https://en.wikipedia.org/wiki/Silicon_controlled_rectifier
During the 50's there may have been a subtle difference between a 4layer p-n-p-n stack referred to as a "solid state current-controlling device" and a 4layer p-n-p-n stacked referred to as an SCR purely to support marketing and trade naming between Bell labs and GE labs but can that be said today? Maybe there was a subtle different between what GE did and what Bell did, but that can be said amongst all silicon dopers. There is no information about what make's an "SCR" have an ornate construct and maybe this was a trade secret, production method but does any exist today? is the original GE "SRC" available to purchase anymore?
Over the years, dealing with academia, suppliers etc... typically I have seen Americans/Japanese use the term SRC while Europeans use the term Thyristor yet in all papers, presentations, discussions, application notes there is nothing imposing a specific structure. Now a GTO has an ornate construct to ensure the gate does not burn out when 1/3 of the forward current is pulled out of it to interrupt the minority carrier flow but that is why they have a unique name.
So... my opinion on this is to merge the two as their usage is synonymous, for the types of queries being seen. The ONLY except to this would be if, and only if there was a discussion about the original GE P-N-P-N part