A proportion is a mathematical comparison between two numbers or ratios that are equal to each other. It is an equation or statement used to depict that two ratios or fractions are equal. Proportions are denoted using the symbol "::" or "=". For example, if there is 1 boy and 3 girls, we could write the ratio as 1:3, and if there are 2 boys and 6 girls, we could write the ratio as 2:6. These two ratios are in proportion because they are equal to each other when simplified to 1:3 and 1:3, respectively.

Proportions are used in various mathematical concepts, including ratios, fractions, and percents. They can be used to solve problems involving scaling, resizing, and mixing. Proportions are closely related to ratios, but they signify the equal relationship between two or more ratios.

In summary, a proportion is a mathematical comparison between two numbers or ratios that are equal to each other, and it is used to solve problems involving scaling, resizing, and mixing.