regulator

1. (genetics) A gene involved in controlling the expression of one or more other genes.

2. (rail transport) A device that controls the supply of steam to the cylinders of a steam locomotive.Examples: "To start No. S.002 effectively with a heavy train, one opens the main regulator sufficiently to move the train off very slowly. An excess of regulator opening with the by-pass valve open can cause heavy slipping; if the worst happens, the slipping continues a little longer after the regulator is closed than on an English locomotive."; "This uncoupling done, Gimbert opened the regulator as Nightall climbed back onto the footplate, in order to pull the one burning wagon away from the rest of the bomb-laden train."

3. (historical) Synonym of bulldozer (“member of intimidating group of white US Southerners”).Synonyms: bulldozerhistorical

4. (mathematics) A positive real number determining the density of the units in Dirichlet's unit theorem.