1Military. A battlefield allowing unrestricted passage in all directions, as opposed to a situation in which one side is defending a stronghold. Frequently in "in (the) open field". Now chiefly historical.

2Figurative. A lack of opposition or competition; an unrestricted or unchallenged opportunity.

3An unenclosed field; specifically a field of arable land without physical division by hedges, ditches, etc., used in common rather than by one owner.

1Of, designating, or relating to a system of agriculture characterized by open fields. Now historical.

2Amer. Football. Of or relating to a field on which the players are widely distributed. Hence of a player: active in the area of the field beyond the line of defence in which the defenders are spread widely apart.