Clyster (also spelled in the 17th Century, `glister') is an old-fashioned word for enema, more particularly for enemas administered using a clyster syringe — that is, a syringe with a rectal nozzle and a plunger. Clyster syringes were used from the 17th century (or before) to the 19th century, when they were largely replaced by enema bulb syringes, bocks, and bags.

The patient was placed in an appropriate position (kneeling, with the buttocks raised, or lying on the side); some servant or apothecary would then insert the nozzle into the anus and depress the plunger, resulting in the liquid remedy (generally, water, but also some preparations) being injected into the colon.

Because of the embarrassment a woman might feel when showing her buttocks (and possibly her genitals, depending on the position) to a male apothecary, some contraptions were invented that blocked all from the apothecary's view except for the anal area. Another invention was syringes equipped with a special bent nozzle, which enabled self-administration, thereby eliminating the embarrassment.

Clysters were administered for symptoms of constipation and, with more questionable effectiveness, stomach aches and other illnesses. In his early-modern treatise, The Diseases of Women with Child, Francis Mauriceau records that both midwives and man-midwives commonly administered clysters to labouring mothers just prior to their delivery.

In Roper's biography of Sir Thomas More, his wife, Thomas More's eldest daughter, fell sick of the sweating sickness and could not be awaked by doctors. After praying, it came to Thomas More "There straightway it came into his mind that a clyster would be the one way to help her, which when he told the physicians, they at once confessed that if there were any hope of health, it was the very best help indeed, much marveling among themselves that they had not afore remembered it." -- Utopia, Thomas More.