Solution:

Hypovolemia is a state of decreased blood volume; more specifically, decrease in volume of blood plasma. It is thus the intravascular component of volume contraction (or loss of blood volume due to things such as bleeding or dehydration), but, as it also is the most essential one, hypovolemia and volume contraction are sometimes used synonymously.

Hypovolemia is characterized by sodium depletion, and thus is distinct from (although often overlapping with) dehydration, excessive loss of body water.