On fire Definition 1 of 3Pro Tip 1/3Urgency Used when an immediate response is required to a dangerous situation. Call 911, the car is on fire! Pro Tip 2/3Alarm Indicates a serious, often destructive incident, prompting quick action. We need to evacuate; the building on fire! Pro Tip 3/3Observation Describes the visual experience of seeing flames engulfing something. I looked outside and saw the field was on fire.