The Collaborative International Dictionary of English v.0.48:

Bloodletting \Blood"let`ting\, n. (Med.)
   The act or process of letting blood or bleeding, as by
   opening a vein or artery, or by cupping or leeches; -- esp.
   applied to venesection.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bloodletting
    n 1: formerly used as a treatment to reduce excess blood (one of
         the four humors of medieval medicine)
    2: indiscriminate slaughter; "a bloodbath took place when the
       leaders of the plot surrendered"; "ten days after the
       bloodletting Hitler gave the action its name"; "the valley is
       no stranger to bloodshed and murder"; "a huge prison battue
       was ordered" [syn: bloodbath, bloodletting, bloodshed,
       battue]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "bloodletting":
   aspiration, bane, bleeding, blood, bloodshed, braining, broaching,
   cupping, dealing death, destruction, destruction of life, dispatch,
   drafting, drainage, draining, drawing, emptying, euthanasia,
   execution, extermination, flow of blood, gore, immolation, kill,
   killing, lapidation, leeching, martyrdom, martyrization,
   mercy killing, milking, phlebotomy, pipetting, poisoning, pumping,
   ritual killing, ritual murder, sacrifice, shooting, siphoning,
   slaughter, slaying, stoning, sucking, suction, taking of life,
   tapping, venesection