An assassin was an individual who was designated to carry out assassination. An assassin's employment varied based on the motivation of the employer; in many cases assassins were hired or acted on behalf of an established or rogue governmental element, or operated independently for hire or in support of their own belief system. According to Odo, "assassins don't like varying their methods." (DS9: "Improbable Cause")
In Klingon culture, the kut'luch was designated as "the ceremonial weapon of an assassin." (TNG: "Sins of the Father") Additionally, according to Miles O'Brien, "Serving on a Klingon ship is like being with a gang of ancient sea pirates. You advance in rank by killing the people above you, so everywhere you turn you're surrounded by potential assassins." (DS9: "Soldiers of the Empire")
Following the assassination of Klingon Chancellor Gorkon aboard the Kronos One in 2293, the crew of the USS Enterprise-A undertook painstaking efforts to discover who the assassins were, as it was believed the Enterprise was the only place they could being hiding.
Later, after James T. Kirk and Leonard McCoy were sentenced to life in prison at Rura Penthe, Kirk was surprised that Martia knew who he was. Her response was that "We don't get many presidential assassins." (Star Trek VI: The Undiscovered Country)
In 2367, a second Klingon Chancellor, K'mpec, was assassinated by poison, given to him in small doses by Duras, son of Ja'rod. K'mpec was aware of the plot, but not who the assassin was, explaining that "the Klingon who kills without showing his face has no honor." Prior to his death, he enlisted Federation Captain Jean-Luc Picard to find his killer. (TNG: "Reunion")
In 2399, Narissa's Romulan assassins were involved in the first attempt to take Dahj Asha in her Greater Boston apartment. They killed her boyfriend Caler and were responsible for her "activation". Later, the second team tried to take Dahj, who was with Jean-Luc Picard at Starfleet Command. This time, they succeeded, as they were ultimately responsible for her death. (PIC: "Remembrance")