Solid State Relay. Colloquially called a "power cube", perhaps due to the company "Silicon Power Cube Inc." that led the production of these devices for many years.
These devices are the power-handling "heart" in many phase control dimmers. They contain two SCRs in a back- to-back configuration, as well as an opto-triac for electrical isolation and sometimes an R/C snubber network for control of dv/dt or "false turn-on" events due to voltage spikes.
Most high-density dimmer systems use dual-SSRs, providing two circuits of dimming. Crydom (the eventual buyer of Silcon Power Cube Inc.) is one of the largest manufacturers.
Shown below in context, the SSR is in the middle of the Colortran D192 dual 20A dimmer module, between the circuit breakers and the two chokes.
An LMI/ETC L86 dual 20amp dimmer.
View attachment 3669
Two units unmounted:
Link to PDFs by Crydom: http://www.crydom.com/en/Tech/Tips/SSR%20the%20inside%20story.pdf and http://www.crydom.com/en/Tech/Tips/How%20to%20test%20an%20AC%20solid%20state%20relay.pdf .