Envy was an emotion characterized by feelings of discontent, longing, or covetousness regarding someone else's advantages, achievements, possessions, or qualities.
In 2268, Dr. Miranda Jones said to Captain James T. Kirk, "Shall I tell you what human companionship means to me? A struggle, a defence against the emotions of others. At times, the emotions burst in on me. Hatred, desire, envy, pity. Pity is the worst of all. I agree with the Vulcans. Violent emotion is a kind of insanity." (TOS: "Is There in Truth No Beauty?")