Trans fats are a type of fat and a type of lipid. They are rare in nature, but are readily produced in an industrial procedure called partial hydrogenation.
In this process, hydrogen gas is passed through oils (made mostly of cis-unsaturated fats), converting some – but not all – of the double bonds to single bonds. The goal of partial hydrogenation is to give the oils some of the desirable properties of saturated fats, such as solidity at room temperature, but an unintended consequence is that some of the cis double bonds change configuration and become trans double bonds. Trans-unsaturated fatty acids can pack more tightly and are more likely to be solid at room temperature. Some types of shortening, for example, contain a high fraction of trans fats.