What is "choy sum"

n. A Chinese vegetable, a variety of (taxlink Brassica rapa parachinensis species noshow=1) subspecies, similar to bok choy but slimmer, with smooth green leaves and pale green stems, and clusters of flowers on the tips of the inner shoots

Choy sum (also spelled choi sum) is a leafy vegetable commonly used in Chinese cuisine. It is a member of the Brassica genus and the Brassicaceae (mustard) family (Brassica rapa var. parachinensis or Brassica chinensis var. parachinensis). Choy sum is a transliteration of the Cantonese name , which can be literally translated as "vegetable stem". It is also known as Chinese Flowering Cabbage.