Definition of Christmas cactus in US English:

Christmas cactus

noun

  • A Brazilian cactus with branching stems of glossy green, flat, broad, tooth-edged sections, the tips of which bear long flowers, typically red or pink, with recurved outer petals. Christmas cacti are widely cultivated as houseplants.

    Schlumbergera bridgesii (or Zygocactus bridgesii), family Cactaceae

    • ‘While poinsettia is the most popular, a Christmas cactus in full bloom is a great gift and easy to care for once the flowers have faded.’
    • ‘You'll be glad to hear that my Christmas cacti are now dripping with pink buds and the geraniums on my frost-free balcony continue to bloom spectacularly.’
    • ‘The best flowers to give at Christmas time are orchids, holly, poinsettias, and the Christmas cactus as well as any red flower.’
    • ‘And remember to get your poinsettias and your Thanksgiving and Christmas cacti ready for well-timed holiday color.’
    • ‘Every fall, my Christmas cactus displays a spectacular profusion of blooms on a covered patio in San Mateo, California.’

Origin

So named because it flowers at about the northern midwinter.