Terminus station on the Metropolitan Line, in Zone 7, opened 2 November 1925, and intended to close in 2016. It was originally intended to extend the line to Watford town centre, and a shop built there was designed to be converted to station use in due course, but this did not happen. There are now plans to extend the line, in a different direction, to Watford Junction Station.
More information on the Wikipedia page 
See also Watford Central Station - the intended terminus of the line.