A taboo was a cultural prohibition.
While on Ominnus in 2266, Leonard McCoy speculated that he and Spock had violated one of the natives' sacred taboos just before they began attacking them for intruding in their garden. (TOS comic: "Child's Play")
The crew of Deep Space 9 thought that the Valtusians had a cultural taboo about being touched, although the Valtusians who visited the station were impersonated by Changelings who wanted to stop a DNA test or blood screening that would have revealed they were not Valtusians. (DS9 novel: The Heart of the Warrior)