Corn syrup is a sweet, viscous liquid made by breaking down corn starch into glucose, used to add sweetness, moisture, and softness to foods while resisting sugar crystallization. Commercial corn syrup production began in the United States in the mid-1800s using acid hydrolysis of corn starch, and the process was later refined with enzymes.
It should not be confused with high-fructose corn syrup, a further-processed version in which some of the glucose is converted to fructose for added sweetness; plain corn syrup (light or dark) is what recipes like pecan pie, caramel, and nougat call for, since it prevents the sugar from turning grainy. It has a mild, neutral sweetness and is also used in marinades and glazes.