A Boiling Water Reactor (BWR) is a member of the class of nuclear reactors cooled by regular water (i.e., not heavy water). There is a single water loop that goes both through the heating of the reactor and the electricity-generating turbine, all of which becomes radioactive in service.
The BWR, with one loop, is smaller than a Pressurized Water Reactor, but is more difficult to service because the turbine is radioactive.