Trans fat: An unhealthy substance that is 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 fatty acids are found in vegetable shortening and in some margarine, crackers, cookies, and snack foods. Trans fatty acids are also found in abundance in many deep-fried foods. Trans fatty acids both raise the 'bad' (LDL) cholesterol and lower the 'good' (HDL) cholesterol levels in blood, markedly increasing the risk of heart disease. Also known as trans fat.