Critical condition was a medical status indicating a high degree of illness or injury, and the possibility of death.
In 1986, Pavel Chekov's condition was critical following a head injury. (Star Trek IV: The Voyage Home)
In 2268, Ensign Rizzo was left in critical condition following an attack by the dikironium cloud creature. (TOS: "Obsession")
In 2372, Harry Kim reported that an adult Kazon male's life signs had been detected and that the Kazon was in critical condition. (VOY: "Basics, Part I")
In 2375, though in critical condition in the IKS Rotarran's sickbay, General Martok was expected to survive. (DS9: "Tacking Into the Wind")