The wide-ranging genus occurs in a variety of open, tropical to subtropical habitats, and is locally dominant. In parts of Africa, Acacias are shaped progressively by grazing animals of increasing size and height, such as gazelle, gerenuk and giraffe. The genus in Africa has thus developed thorns in defence against such herbivory. They belong to the subfamily Mimosoideae, the major clades of which may have formed in response to drying trends and fire regimes that accompanied increased seasonality during the late Oligocene to early Miocene (25 mya). Pedley (1978), following Vassal (1972), viewed Acacia as comprising three large subgenera, but subsequently (1986) raised the rank of these groups to genera Acacia, Senegalia (s.l.) and Racosperma, which was underpinned by later genetic studies. The International Code of Nomenclature provides that under the rules, if Acacia is dismantled, then the name Acacia follows the type.Acacia (/keɪ/ or /keɪsi/) is a monophyletic genus of flowering plants in the legume family, Fabaceae, commonly known as thorn trees or shittah trees. The genus name is derived via Latin from ancient Greek ακακία (akakia). It was the name used by Theophrastus and Dioscorides to denote thorn trees, the word root being κί (akis) or κή (ak), meaning "thorn" and "point" respectively. Before discovery of the New World, Europeans in the Mediterranean region were familiar with several species of Acacia, which they knew as sources of medicine, and had names for them that they inherited from the Greeks and Romans.
They are trees or shrubs, sometimes climbing, and are always armed. Younger plants, especially, are armed with spines which are modified stipules, situated near the leaf bases. Some (cf. A. tortilis, A. hebeclada, A. luederitzii and A. reficiens) are also armed with paired, recurved prickles (in addition to the spines). The leaves are alternate and bipinnately arranged, and their pinnae are usually opposite. The racemose inflorescences usually grow from the leaf axils. The yellow or creamy white flowers are produced in spherical heads, or seldom in elongate spikes, which is the general rule in the related genus Senegalia. The flowers are typically bisexual with numerous stamens, but unisexual flowers have been noted in A. nilotica (cf. Sinha, 1971). The calyx and corolla are usually 4 to 5-lobed. Glands are usually present on the rhachis and the upper side of the petiole. The seed pod may be straight, curved or curled, and either dehiscent or indehiscent.
Of the 163 species, 52 are native to the Americas, 83 to Africa, Madagascar and the Mascarene Islands, 32 to Asia and 9 to Australasia and the Pacific Islands.
The following species are suspected to belong to Acacia.