Wiktionary
pentagrid converter
n. A compound thermionic valve used in superheterodyne radio receivers.
Wikipedia
Pentagrid converter
The pentagrid converter is a type of radio receiving valve ( vacuum tube) with five grids used as the frequency mixer stage of a superheterodyne radio receiver.
The pentagrid was part of a line of development of valves that were able to take an incoming RF signal and change its frequency to a fixed intermediate frequency, which was then amplified and detected in the remainder of the receiver circuitry. The device was generically referred to as a frequency changer or just mixer.