An antenna designed with a pattern which favours some directions more than others.
Antennas having one or two major lobes are sometimes referred to as unidirectional or bidirectional antennas respectively. The lobes of bidirectional antennas are usually oppositely directed.
Reference: B.S. (modified)
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
Centre of buoyancy
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM