Margin is the space between our load and our limits. It is the amount allowed beyond that which is needed. It is something held in reserve for contingencies or unanticipated situations. Margin is the gap between rest and exhaustion, the space between breathing freely and suffocating. (Quoted from Margin by Richard Swenson)
Although some people seem capable of thriving without margin, most of us find it a prerequisite for well being. Margin grants freedom and permits rest. It nourishes both relationship and service. Spiritually, it allows availability for the purposes of God.