The story
Initialism from the Russian name: "реа́ктор большо́й мо́щности кана́льный" (reaktor bolshoy moshchnosti kanalnyy), meaning "high-power channel-type reactor".
Etymology from Wiktionary, CC BY-SA 4.0.
RBMK
Today it means: A kind of graphite-moderated nuclear reactor, designed so that, instead of a large steel pressure vessel surrounding the entire core, the core is surrounded by a cylindrical annular steel tank inside a concrete vault and each fuel assembly is enclosed in an individual pipe or channel that also contains the coolant. Full definition →