| genus abelia | (n) chiefly east Asian shrubs |
| genus acacia | (n) large genus of shrubs and trees and some woody vines of Central and South America, Africa, Australia and Polynesia: wattle; mimosa |
| genus acanthus | (n) bear's breeches |
| genus achillea | (n) perennial often aromatic and sometimes mat-forming herbs of north temperate regions: yarrow; milfoil |
| genus achimenes | (n) genus of tropical perennial American herbs |
| genus actinomyces | (n) type genus of the family Actinomycetaceae |
| genus addax | (n) genus of antelopes of northern African deserts |
| genus aegyptopithecus | (n) a genus of Hominoidea |
| genus aepyornis | (n) type genus of the Aepyornidae: elephant birds |
| genus aerides | (n) epiphytic orchids of tropical Asia having stiff leaves and fragrant white flowers in arching racemes |
| genus aeschynanthus | (n) large genus of East Indian ornamental woody epiphytic plants |
| genus afropavo | (n) Congo peafowl |
| genus agama | (n) type genus of the Agamidae |
| genus agapanthus | (n) small genus of South African evergreen or deciduous plants; sometimes placed in the family or subfamily Alliaceae |
| genus agave | (n) type genus of the Agavaceae; in some classifications considered a genus of Amaryllidaceae |
| genus ageratum | (n) genus of tropical American herbs grown for their flowers |
| genus agrimonia | (n) genus of herbs found chiefly in north temperate regions having pinnate leaves and yellow flowers followed by bristly fruit |
| genus ailanthus | (n) small genus of east Asian and Chinese trees with odd-pinnate leaves and long twisted samaras |
| genus albizia | (n) large genus of unarmed trees and shrubs of Old World tropics, Syn. genus Albizzia |
| genus albuca | (n) genus of bulbous plants of South Africa; sometimes placed in subfamily Hyacinthaceae |
| genus allamanda | (n) genus of tropical American woody vines |
| genus alligator | (n) type genus of the Alligatoridae |
| genus allosaurus | (n) carnivorous dinosaur of North America; late Jurassic, Syn. genus Antrodemus |
| genus alocasia | (n) tropical Asiatic herbs similar to Colocasia but distinguished by a large sterile spadix |
| genus aloe | (n) large genus of chiefly African liliaceous plants; in some systems placed in family Aloeaceae |
| genus alstroemeria | (n) genus of showy South American herbs with leafy stems; sometimes placed in family Alstroemeriaceae or in family Amaryllidaceae |
| genus althaea | (n) hollyhocks; in some classification systems synonymous with genus Alcea |
| genus amaryllis | (n) type genus of the Amaryllidaceae; bulbous flowering plants of southern Africa |
| genus ambrosia | (n) comprising the ragweeds; in some classification considered the type genus of a separate family Ambrosiaceae |
| genus ammobium | (n) small genus of Australian herbs grown for their flowers |
| genus amoeba | (n) protozoan inhabiting moist soils or bottom vegetation in fresh or salt water |
| genus amorpha | (n) American herbs or shrubs usually growing in dry sunny habitats on prairies and hillsides |
| genus amorphophallus | (n) genus of large tropical east Asian cormous aroids: devil's tongue; snake palm |
| genus amphioxus | (n) type genus of the Amphioxidae |
| genus amphiuma | (n) congo snakes |
| genus anatotitan | (n) genus of large duck-billed dinosaurs; late Cretaceous |
| genus anchusa | (n) rough and hairy Old World herbs |
| genus andrena | (n) a solitary burrowing short-tongued bee |
| genus andromeda | (n) low-growing shrubs of northern regions of northern hemisphere |
| genus andryala | (n) a genus of hardy hairy latex-producing perennials of Mediterranean area |
| genus anemone | (n) perennial herbs with tuberous roots and beautiful flowers; of north and south temperate regions |
| genus angelica | (n) biennial or perennial herbs of the northern hemisphere; have a taproot |
| genus angiopteris | (n) one species: tree fern |
| genus angrecum | (n) genus of tropical Old World epiphytic orchids with showy flowers sometimes grotesque, Syn. genus Angraecum, Angraecum |
| genus anhinga | (n) type genus of the Anhingidae |
| genus ankylosaurus | (n) armored herbivorous dinosaurs of the Cretaceous |
| genus anoa | (n) a genus of mammals of the family Bovidae |
| genus anomalops | (n) type genus of the family Anomalopidae |
| genus anomalopteryx | (n) small moas |
| genus anthurium | (n) large genus of often epiphytic evergreen tropical American plants often cultivated as houseplants |
