Invisibility was the ability of an object or individual to remain unseen, either by other individuals or sensors. This form of stealth or camouflage could be achieved via stealth technology or biological ability.
A cloaking device was a technology used to obtain invisibility, which could allow one to avoid detection from sensors. Romulans were one such species who utilized cloaking devices, on their Bird-of-Prey warships. (TOS: "Balance of Terror"; SNW: "A Quality of Mercy")
The Tosk species could make themselves invisible but still appeared to sensors. (DS9: "Captive Pursuit")