Proportions refer to the relationship between two quantities, showing how much of one thing there is compared to another. For example, if a recipe calls for 2 cups of flour for every 1 cup of sugar, the proportion of flour to sugar is 2:1. This concept helps in scaling recipes, creating models, and understanding ratios in various fields. In mathematics, proportions can be expressed as equations, such as a/b = c/d, where a and b are one pair of quantities, and c and d are another. Proportions are useful in solving problems involving similar shapes, like triangles, where corresponding sides maintain the same ratio.