A legal ward was a minor child who had been transferred into the custody of another who was known as a legal guardian.
After her parents were killed, Michael Burnham became a ward of Sarek. (DIS: "Battle at the Binary Stars")
In 2269, James T. Kirk ordered Nyota Uhura to run a computer check on Rayna Kapec, who Flint claimed was his legal ward after the death of her parents. (TOS: "Requiem for Methuselah")
Su'Kal became Saru's ward after he was rescued from Theta Zeta in 3189. (DIS: "Rubicon")