What is "router plane"

n. a woodworking plane with a narrow cutting head that will make grooves with smooth bottoms

A router plane is a plane used for smoothing out sunken panels, and more generally for all depressions below the general surface of the pattern.

It planes the bottoms of recesses to a uniform depth and can work into corners that could be reached only by the use of a chisel.

Nowadays, it is often replaced by the electrical router but continues to be preferred in special cases.