A 'No-Fly ZONE' is an area of airspace in which aircraft are not allowed to fly. It is typically established by a government or international organization in order to protect civilians from military action or to prevent unauthorized flights. The term 'No-Fly ZONE' can also refer to a geographical area on the ground, such as a restricted military base or a protected wildlife sanctuary. The crossword solution for this term is 'ZONE', which is a four-letter word meaning an area or region.