Ritual sacrifice was the practice of killing a lifeform to serve as a religious offering.
Sacrifice of animals[]
In Klingon culture, the traditional method for making Var'Hama candles from targ shoulders required three of the animals from the Hamar Mountains to be ritually sacrificed at dawn. (DS9: "You Are Cordially Invited")
A traditional Klingon wedding ceremony involved the ritual sacrifice of a targ before the wedding feast. (DS9: "A Time to Stand")
Sacrifice of sentient beings[]
The D'Arsay culture included ritual sacrifice. In their mythology, Ihat constantly sought to escape being put on Masaka's sacrificial slab. (TNG: "Masks")
On ancient Qo'noS, live sacrifices were offered to the tyrant Molor at his shrines, built at the seven entrances to the planet's underground volcano system. (DIS: "Will You Take My Hand?")
The harvesting of Kelpiens by the Ba'ul in the mid-23rd century was codified in Kelpien culture as a ritual sacrifice and overseen by priests. (ST: "The Brightest Star")
In 2267, when "Lord" Garth of Izar wondered if the unenthusiastic "heir apparent" James T. Kirk might want a larger role in his coronation ceremony, he suggested Human sacrifice instead. After Kirk graciously declined ("No, I wouldn't enjoy that at all."), saying Garth needed him alive, the insane man agreed, saying he had other candidates. (TOS: "Whom Gods Destroy")
On Kelis' homeworld, the inhabitants once sacrificed a person every year in honor of winter. At some point, for forgotten reasons, the sacrifice was replaced with a play, and the temple in which the sacrifice took place was converted into a theater. (VOY: "Muse")
Child sacrifice[]
Child sacrifice was the practice of killing a child and offering it to a god.
In 2376, attempting to dissuade B'Elanna Torres' plan to voluntarily return to the Barge of the Dead to see her mother again, Captain Kathryn Janeway explained that there was a limit to freedom of worship aboard USS Voyager, giving the hypothetical example of Torres wishing to sacrifice a child to the gods as a comparison. Torres found the analogy to be absurd. (VOY: "Barge of the Dead")