An emergency was an unexpected and dangerous situation which occurred suddenly and required quick action to deal with it. Emergencies could bring harm to health, life, property, or environment.
In the 23rd and 24th centuries, the commanding officer of a Federation starship could call the crew to emergency stations during an emergency. (TOS: "Where No Man Has Gone Before"; TNG: "The Wounded"; VOY: "Once Upon a Time")
In 2265, Doctor Elizabeth Dehner was studying crew reaction to emergency conditions aboard the USS Enterprise. (TOS: "Where No Man Has Gone Before")