Physiol. the barrier created by the walls of the capillaries of the brain that prevents certain substances, as most proteins and drugs, from passing from the blood into the brain tissue and cerebrospinal fluid
See blood-brain barrier in American Heritage Dictionary 4
noun
A physiological mechanism that alters the permeability of brain capillaries, so that some substances, such as certain drugs, are prevented from entering brain tissue, while other substances are allowed to enter freely.