Trans fat: An unhealthy substance, also known as trans fatty acid, made through the chemical process of hydrogenation of oils. Hydrogenation solidifies liquid oils and increases the shelf life and the flavor stability of oils and foods that contain them. Trans fat is found in vegetable shortenings and in some margarines, crackers, cookies, snack foods and other foods. Trans fats are also found in abundance in french fries.. To make vegetable oils suitable for deep frying, the oils are subjected to hydrogenation, which creates trans fats. Among the hazards of fast food, fries are prime in purveying trans fats.