cactus(n = noun.plant) - any succulent plant of the family Cactaceae native chiefly to arid regions of the New World and usually having spines;
is a kind of succulent
is a member of cactaceae, cactus family, family cactaceae
has particulars: acanthocereus pentagonus, acanthocereus tetragonus, pitahaya, pitahaya cactus, aporocactus flagelliformis, rat's-tail cactus, rattail cactus, ariocarpus fissuratus, living rock, carnegiea gigantea, saguaro, sahuaro, night-blooming cereus, coryphantha, barrel cactus, echinocactus, hedgehog cereus, rainbow cactus, epiphyllum, orchid cactus, barrel cactus, easter cactus, hatiora gaertneri, schlumbergera gaertneri, night-blooming cereus, chichipe, lemaireocereus chichipe, lophophora williamsii, mescal, mezcal, peyote, mammillaria, feather ball, mammillaria plumosa, garambulla, garambulla cactus, myrtillocactus geometrizans, knowlton's cactus, pediocactus knowltonii, nopal, prickly pear, prickly pear cactus, cholla, opuntia cholla, mistletoe cactus, christmas cactus, schlumbergera baridgesii, schlumbergera buckleyi, night-blooming cereus, crab cactus, schlumbergera truncatus, thanksgiving cactus, zygocactus truncatus

   Any plant of the order Cactacæ, as the prickly pear and the night-blooming cereus. See Cereus. They usually have leafless stems and branches, often beset with clustered thorns, and are mostly natives of the warmer parts of America. [1913 Webster]

cactus, n. (pl. cacti or cactuses) any succulent plant of the family Cactaceae, with a thick fleshy stem, usu. spines but no leaves, and brilliantly coloured flowers.

Idiom:
cactus dahlia any kind of dahlia with quilled petals resembling a cactus flower.

Derivative:
cactaceous adj.

Etymology:
L f. Gk kaktos cardoon

barrel cactus, cactus euphorbia, cactus family, cactus mouse, cactus wren, christmas cactus, crab cactus, easter cactus, garambulla cactus, golden barrel cactus, hedgehog cactus, knowlton's cactus, mistletoe cactus, orchid cactus, pitahaya cactus, prickly pear cactus, rainbow cactus, rat's-tail cactus, rattail cactus, redbird cactus, russian cactus, thanksgiving cactus, vine cactus