From meta (between) and aichmē (spear point). Originally the space between two armies — the no-man's land between opposing forces. By extension, any interval or intermediate space.
Though rare in the NT, this concept of the space between opposing forces resonates with the spiritual warfare Paul describes. Believers live in the interval between Christ's first and second coming — already redeemed but not yet glorified, in enemy territory but more than conquerors. The Christian life is lived in the 'already but not yet,' the space between promise and fulfillment.