What does a thyristor do in a circuit?

A thyristor is a solid state three terminal electronic power switch with two power terminals which carry the operational part of the device current and one gate or trigger terminal which is used to switch on the thyristor when required.

Whenever the current flowing though it changes diirection or simply falls to zero, a thyristor will switch off.

Unlike a transistor, after its gate/trigger signal is removed a thyristor will remain switched on for as long as the direction of the current flowing through it remains the same, even if the amperage of that current changes.

Unlike a mechanical switch the thyristor has operational supply polarities ( positive & negative of the supply ) which have to be observed. Therefore it can switch on and conduct for one supply polarity only.

Since it is a solid state device a thyristor can operate at much faster switching speeds than any electro-mechanical switch.

With a DC supply of correct polarity, a thyristor will conduct when triggered but will not then switch off till the current is interrupted by some other means such as removal of the supply or by use other, much more complex, circuitry which forces the current to flow in the reverse direction.

With an AC supply, when triggered a thyristor will conduct when the supply is of correct polarity and will then automatically switch off when the AC supply polarity reverses.

For controlled operation in both directions, using the gate as a trigger on the both the forward and the reverse part of the AC Supply, a second thyristor connected with reverse polarity has to be used in parallel with the first one. This idea is basically what is used to make AC lamp dimmers nowadays.

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A thyristor is a transistor having a thyratron-like characteristic; as collector current is increased to a critical value, the alpha (amplification factor) of the unit rises above unity to give a high-speed triggering action.

That description just given (now shown in italics) is wrong! It is not for a thyristor but for a thermistor!

A thermistor is a device with 2 wires coming out of it which changes its resistance to electrical current flow as its temperature is changed. They are used in devices that have to perform some job based on the temperature of the medium they are in.

The thyristor is a solid-state semiconductor device with four layers of alternating N- and P-type material. They act as bi-stable switches, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward biased. (That is, for as long as the voltage across the device has not reversed).

Some sources define silicon controlled rectifiers and thyristors as synonymous. Other sources define thyristors as a larger set of devices with at least four layers of alternating N and P-type material.

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Re. the error shown in italics above... The first answer at the top of the page is correct: a thyristor (aka SCR or Silicon Controlled Rectifier) is a four layer solid-state switch, and a thermistor is a temperature-sensitive device consisting of a material with a high thermal coefficient of resistance, either positive or negative.

Thyristors are solid-state semiconductor devices with four layers of alternating N- and P-type material. They act as bi-stable switches, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward-biased (That is, as long as the voltage across the device has not reversed).