obstacle free zone (OFZ)

accident-free flying / mishap-free flying

mishap-free flying / accident-free flying

free fire zone

nuclear-free zone

weapons free zone (WFZ)

grid zone / grid-zone

zone standard time (zone time) (ZST)

killing zone / kill zone

grid zone designation / Grid Zone Designator

grey-zone / grey zone (international relations)

hundred-per-cent zone (one-hundred-per-cent zone)

no-fire zone / no fire zone

flight restriction zone / flight restricted zone (FRZ)

antiboat obstacle

antipersonnel obstacle

antipersonnel obstacle breaching system (APOBS)

antitank obstacle

antivehicular obstacle

artificial obstacle

barbed wire obstacle

beach obstacle

composite obstacle

concrete obstacle

demolish an obstacle

incendiary obstacle

log obstacle

man-made obstacle

mine obstacle

natural obstacle

obstacle belt/ course

obstacle development

obstacle race