Real Dictionary

control rod in American English
an adjustable rod or bar containing a neutron-absorbing material or, in a fast reactor, fuel used to regulate the fission chain reaction inside a nuclear reactor core
noun: a neutron-absorbing material, as boron or cadmium, in the shape of a rod or other configuration, that can be moved into or out of the core of a nuclear reactor to regulate the rate of fission

control rod in British English
noun: one of a number of rods or tubes containing a neutron absorber, such as boron, that can be inserted into or retracted from the core of a nuclear reactor in order to control its rate of reaction