Vasodilation was the widening of blood vessels, resulting from relaxation of smooth muscle cells within the vessel walls.
In 2153, Doctor Phlox discovered mild vasodilation in Hoshi Sato's prefrontal cortex, which explained headaches Sato was experiencing. (ENT: "Exile")
In 2373, Tova Veer noticed that there was no sign of vasodilation in the skin of Tom Paris and B'Elanna Torres during their social interaction, while he and Professor Gegen covertly observed them aboard the USS Voyager. (VOY: "Distant Origin")