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]