An eyebrow or brow was an area above one's eye, usually accented by hair. Both Romulans and Vulcans had slanted eyebrows.
In 2153, Lieutenant Talas cautioned Malcolm Reed that he might singe his eyebrows when he brought the EPS synchronizer back online. (ENT: "Proving Ground")
In 2268, Philana gave Spock a kithara, saying it was "to soothe his ever-active brow." (TOS: "Plato's Stepchildren")
In 2371, an immobilized Neelix asked The Doctor to scratch an itch above his left eyebrow. (VOY: "Phage")
Eyebrows as "tell"sEdit
A raised eyebrow could be a sign of interest or incredulity. (DS9: "Emissary") Spock frequently raised his eyebrows when something unexpected happened.
In 2368, when asked to explain her good luck at poker, Beverly Crusher teased William T. Riker, saying his left eyebrow rose when he was bluffing, when in fact it didn't. (TNG: "Cause and Effect")
In 2373, the phony Keiko O'Brien noted that Miles O'Brien always tightened
his brow just before he asked a question, saying it was something only a wife would notice. (DS9: "The Assignment")