A fatty acid whose carbon chain can absorb additional hydrogen atoms WordNet 3.0 & Open English WordNet

An unsaturated fat is a fat or fatty acid in which there is at least one double bond within the fatty acid chain. Wikipedia

A fat or oil, from either animal or vegetable sources, containing a high proportion of unsaturated fatty acids; will typically be liquid at room temperature; a diet high in unsaturated rather than saturated fats is thought to contribute to lower levels of cholesterol in the blood. Wiktionary

Unsaturated fat. Wiktionary (translation)

A fatty acid, such as oleic acid, that contains at least one carbon to carbon double bond. Wiktionary

A fatty acid that contains at least one carbon to carbon double bond. Wiktionary (translation)