nonparasitic plant that uses another plant as a host for mechanical support. but does not derive nutrients or water from it (contrast with parasite and saprophyte).

Merriam-Webster Online Dictionary
epiphyte (noun)
a plant that derives its moisture and nutrients from the air and rain and grows usually on another plant
epiphyte (Wikipedia)
Tillandsia bourgaei growing on an oak tree in Mexico

An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. Epiphytes take part in nutrient cycles and add to both the diversity and biomass of the ecosystem in which they occur, like any other organism. They are an important source of food for many species. Typically, the older parts of a plant will have more epiphytes growing on them. Epiphytes differ from parasites in that they grow on other plants for physical support and do not necessarily affect the host negatively. An epiphytic organism that is not a plant is sometimes called an epibiont. Epiphytes are usually found in the temperate zone (e.g., many mosses, liverworts, lichens, and algae) or in the tropics (e.g., many ferns, cacti, orchids, and bromeliads). Epiphyte species make good houseplants due to their minimal water and soil requirements. Epiphytes provide a rich and diverse habitat for other organisms including animals, fungi, bacteria, and myxomycetes.

Epiphyte is one of the subdivisions of the Raunkiær system. The term epiphytic derives from the Greek epi- (meaning 'upon') and phyton (meaning 'plant'). Epiphytic plants are sometimes called "air plants" because they do not root in soil. However, there are many aquatic species of algae that are epiphytes on other aquatic plants (seaweeds or aquatic angiosperms).

Epiphytes can grow on the trunks of trees or sometimes in the canopy of a tree
An epiphytic orchid on a tree in a Brazilian cloud forest
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