A slang term for a red blood cell transfusion made up of 'packed' red blood cells (PRBC). When donors give blood, the blood bank concentrates the red blood cells in a small volume of plasma. The excess plasma is saved for transfusion to those who need plasma. The concentrated red blood cells are called 'packed' because they are in a more compressed volume of blood than they were in the donor's blood.
The above paragraph was taken from www.babyzone.com
This type of blood transfusion is usually done to raise hematocrit levels in patients who are deficient for one reason or another (like anemia).