A radical derived from a phosphatidic acid by the loss of a hydrogen atom from the phosphate group. Usually attributive or a formative element in the names of esters (phospholipids) occurring in cell membranes, in which a radical formed by the loss of a hydroxyl group from the compound denoted by the second element is attached to the phosphate group, as "phosphatidylinositol", "phosphatidylserine", etc.