Embedded system
A computer built into a device with a single job, which nobody using it would ever call a computer.
The board in a washing machine, a car's ABS unit, a pacemaker and a home router are all embedded systems. What they share is hardware sized for the task and not a byte more: a little RAM, a few hundred kilobytes of flash, a power budget that has to last years on a coin cell, and often a hard deadline for responding. The software is called firmware and runs either on bare metal or on top of a small real-time operating system.
The Apollo Guidance Computer of the 1960s is usually cited as the first modern example. Today far more processors ship inside embedded systems than inside anything sold as a computer, and most of them will never receive a security update — which is how "IoT" became a swear word in the security industry.