A plant is dioecious if male and female forms exist in separate individuals.

Indeed, the word translates roughly to "two houses", which makes a useful mnemonic. About 6% of the world's flowering plants (i.e. angiosperms) are dioecious. Dioecy is believed to convey an evolutionary advantage to a species because it helps prevent inbreeding; a dioecious plant cannot fertilize itself. Thus, a dioecious population can avoid inbreeding depression, the exposure of harmful mutations. Despite this hypothesized advantage, dioecy is less frequent than one would be led to believe.

See also: gynodioecious, monoecious.

Reference: Heilbuth JC. 2000. American Naturalist 156: 221-241.