Cascara Sagrada Bark (Rhamnus purshiana), also known as Cascara Buckthorn, Cascara, Bearberry, and in the Chinook Jargon, Chittam or Chitticum, is the largest species of buckthorn, occasionally growing up to 15 m tall, though more commonly a large shrub or small tree 5-10 m tall, with a trunk 20-50 cm in diameter. The bark is brownish to silver-grey with light splotching. The leaves are deciduous, alternate, clustered near the ends of twigs.

The dried, aged bark of this tree has been used continually for at least 1,000 years by both native and immigrant Americans as a laxative natural medicine, commercially called "Cascara Sagrada", but old timers call it "chitticum bark".