Blood plasma is the liquid in which blood cells are suspended.
When Dr. McCoy and Nurse Chapel were preparing for surgery on Sarek for his heart, an operation that" would require tremendous amounts of blood for the patient", Chapel informed McCoy that she had checked their stores and told him that there was not enough Vulcan blood or plasma aboard the USS Enterprise for the operation, forcing them to look for alternate sources. (TOS: "Journey to Babel")
In 2371 during surgery on Bareil Antos, Dr. Bashir was forced to increase the plasma levels due to the loss of arterial pressure and the cardiac arrest issues after Bareil began hemorrhaging. (DS9: "Life Support")
When Jake Sisko and Dr. Bashir found themselves on Ajilon Prime in the middle of a war zone in 2373, a Bolian orderly and Human nurse were looking for blood plasma for transfusions as they cared for the injured from the attacks by the Klingons. (DS9: "Nor the Battle to the Strong")