Definition
The estimated time at which the aircraft will commence movement associated with departure, as defined by ICAO. It is the time the aircraft is expected to start moving back from the parking position (push-back or taxi out of the gate or stand), not the expected takeoff time. EOBT is filed in Item 13 of the ICAO flight plan.
Plain English
The time you expect the aircraft to first start moving away from its parking spot at the start of the flight.
Context Anchor
Seen in flight planning and air traffic planning, especially for departures using international procedures.
Derivation
Off-block' refers to the moment the chocks are removed from the wheels and the aircraft begins to move from its parked position. Historically, wooden blocks (chocks) were placed against the tires to keep a parked aircraft from rolling. 'On-block' means parked with chocks in; 'off-block' means chocks pulled and ready to move. So 'estimated off-block time' literally means 'the time we expect the chocks to come off.'
Why Pilots Care
Accurate EOBT keeps departure slots valid and prevents delays from ATC flow management.
Intuition Check
Do not read this as the estimated takeoff time. It means the expected time the aircraft starts moving away from parking for departure.
Example Sentence 1
The crew filed an EOBT of 1430Z, planning a 15-minute taxi to the active runway.
Example Sentence 2
ATC used the filed estimated off-block time to assign a departure slot.