Jelly in peanut butter and jelly sandwiches is a sweet, spreadable fruit preserve made by cooking fruit juice (or crushed fruit) with sugar until it gels into a thick, fruity spread. The most common varieties are grape jelly and strawberry jelly, though raspberry, blackberry, and other fruit flavors are popular too.
The term "jelly" in American usage refers specifically to a smooth, clear preserve made from strained fruit juice without pulp or fruit pieces. This differs from jam, which contains fruit pulp and has a chunkier texture, and from preserves, which have whole or large fruit pieces. The gelatin-like consistency comes from pectin, a natural carbohydrate in fruit that causes the mixture to thicken and set when combined with sugar and heat.
Peanut butter and jelly sandwiches became iconic in American cuisine, particularly as a school lunch staple. The combination gained popularity in the early 20th century and was even included in military rations during World War II. The sweet jelly complements the savory, protein-rich peanut butter, creating a balanced flavor profile that appeals to both children and adults. While grape jelly remains the classic choice, modern variations use different jellies, jams, and spreads to suit individual preferences. Some regions and countries have different names for similar products—what Americans call "jelly" might be called "jam" in other English-speaking countries, though the distinction between smooth and chunky versions generally remains.