One of two kinds of plants on which a parasitic fungus (e.g., a rust) must develop to complete its life cycle. . 
a secondary host that becomes infected and is necessary for alternating generations of a disease-causing organism.
One of the two hosts of a pathogen where different stages of the life cycle occur on unrelated hosts. When several or many plant species are hosts of a given pathogen they are known as alternative hosts.
One of the two dissimilar plants infected by a heterocyclic fungus, i.e., rust fungi that use two hosts to complete a life cycle.
a host, other than the usual or primary host, on which a parasitic organism can sustain itself.