1. The edge or border; the edge of a body of water where it meets the land. As applied to a boundary line of land, the “margin” of a river, creek, or other water course means the center of the stream. But In the case of a lake, bay, or natural pond, the “margin” means the line where land and water meet. 2. A sum of money, or its equivalent, placed in the hands of a stockbroker by the principal or person on whose account the purchase is to be made, as a security to the former against losses to which he may be exposed by a subsequent depression in the market value of the stock.