Orchids are plants that belong to the family Orchidaceae (/ˌɔːrkɪˈdeɪsi.iː, -si.aɪ/), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant. Orchids are cosmopolitan plants, living in diverse habitats on every continent except Antarctica. The world's richest diversity of orchid genera and species is in the tropics. Many species are epiphytes, living on trees. The flowers and their pollination mechanisms are highly specialized, attracting insect pollinators by colour, pattern, scent, pheromones, and sometimes by mimicking female insects. Orchids have very small seeds, relying on fungal partners for germination. Some orchids have no leaves, either photosynthesizing with their roots or relying entirely on fungal partners for food.

Orchidaceae is one of the two largest families of flowering plants. It contains about 28,000 currently accepted species in around 700 genera. That represents some 6–11% of all species of seed plants. Horticulturists run many orchid societies around the world; they have produced many hybrids and cultivars.

The type genus (i.e. the genus after which the family is named) is Orchis. The genus name comes from the Ancient Greek ὄρχις (órkhis), literally meaning "testicle", because of the shape of the twin tubers in some species of Orchis. The term "orchid" was introduced in 1845 by John Lindley in School Botany, as a shortened form of Orchidaceae. In Middle English, the name bollockwort was used for some orchids, based on "bollock" meaning testicle and "wort" meaning plant.

Orchids are easily distinguished from other plants, as most of them share some very evident derived characteristics or synapomorphies. Among these are: bilateral symmetry of the flower (zygomorphism), many resupinate flowers, a nearly always highly modified middle petal (labellum), stamens and carpels fused into a column, and extremely small seeds.

All orchids are perennial herbs that lack any permanent woody structure. They can grow according to two patterns:

Terrestrial (ground-living) orchids may have rhizomes or tubers. Epiphytic orchids, those that grow upon a support, have aerial roots. The older parts of the roots are covered with a velamen, a layer of dead cells.

Like most monocots, orchids usually have simple (untoothed) leaves with parallel veins. Some orchids such as Vanda are distichous, with their leaves arranged in two ranks on opposite sides of the stem. This is the arrangement at the base of all orchid shoots, though in many species the leaves higher up the shoot switch to a spiral phyllotaxis. Orchids are perennial; most species add new leaves at the apex while the oldest leaves gradually die off, but some such as Catasetum shed their leaves annually in the dry season, developing new leaves and new pseudobulbs each year.

Some epiphytic orchids, such as Taeniophyllum aphyllum, are leafless, depending on their green roots for photosynthesis. Other epiphytes like Phalaenopsis have leaves, but rely on photosynthesis in their green roots to prevent hypoxia (lack of oxygen) of the roots. Orchids of the genus Corallorhiza (coralroot orchids) have no leaves; instead they have symbiotic or parasitic associations with fungal mycelium, though which they absorb sugars.