In restricted sense a hybrid is a cross between two genetically different (homozygous) inbred lines. Such a cross is also called a “F1-hybrid” or a single cross.
Products of a series of other crosses also may be called hybrids. In some plant species, it is not possible to create completely homozygous lines: due to inbreeding depression, they lose their fitness after a few selfings. Hybrids are then made by crossing lines that only have been selfed for a few generations.
Examples: leek, cichory.