A classification of fish based on fat content, describing species whose flesh contains between 2 and 6 percent fat or oil. Medium oily fish sit between lean white fish and rich oily fish like mackerel and salmon. Examples include sea bass and some trout, offering a balance of delicate flavour with enough natural oil to stay moist during cooking. The fat in fish is largely unsaturated, including beneficial omega-3 fatty acids, though at lower levels than in fully oily species. This classification helps nutritionists, caterers, and consumers compare fish for dietary planning, cooking methods, and menu labelling.