Light corn syrup is a clear-colored syrup made by extracting sugars from cornstarch. It also often includes a little vanilla flavoring. You'll find it listed as an ingredient in recipes for candy, caramel popcorn, frosting, and sweet sauces. It doesn't crystallize like sugar so it can withstand high cooking temperatures.
If you don't have a bottle in the pantry, you might not need to run to the store. There are a number of light corn syrup substitutes that should work out just fine; which you choose will depend on you're making.
Light Corn Syrup Substitutes
You can replace 1 cup of corn syrup with one of these simple substitutes:
Which Will Work Best in Your Recipe?
Corn syrup doesn't add flavor to a recipe—just sweetness—so sugar is definitely the best substitute in terms of flavor. By dissolving it in water, you're creating a concentrated simple syrup. This liquid form is a great substitute for pies (like pecan) and coated fruits. However, unlike corn syrup, sugar crystallizes at high temps, so it's not a good choice for candy recipes that you have to take past the softball stage (235 F).
Assuming your recipe can take the addition of maple flavoring, any maple-flavored syrup is a brilliant substitute in all situations. It's made from corn syrup, so really you're just substituting like for like when you go this route. Of course, this is not a solution if you're allergic to corn or trying to cut corn syrup from your diet.
Honey is a good stand-in in baked good recipes because it will retain the moisture. While the honey will add a bit of flavor, the difference will be subtle as long as you stick with light-colored honey.
Agave syrup (or agave nectar) has a fairly mild flavor, so it's another option to consider. It can be used in pies, sauces, and other dessert recipes, but won't work for candy making.
Light molasses is perhaps the substitute of last resort. It will give you the body you're after, but will also change the flavor of your recipe more than any of the other substitutes. Avoid using blackstrap molasses at all costs. It has a very distinct taste that is likely to overwhelm the other flavors in your recipe.
If you're making a candy that has to go to the hardball stage, corn syrup, brown rice syrup, or another glucose syrup really is the best way to go. Hard candies were made long before corn syrup existed, so consider consulting a vintage cookbook for techniques if you're determined to forgo corn syrup entirely.
If you live outside the United States and haven't been able to find corn syrup locally, glucose syrup (also known as confectioner's glucose) or golden syrup can be used in its place. In fact, corn syrup is just a type of glucose syrup. It can also be made from potatoes, wheat, rice, and a few other things.