A proportional relationship is a relationship between two quantities where their ratio remains constant. This means that if one quantity increases or decreases, the other quantity changes in a way that keeps the ratio the same. For example, if you have 2 apples for every 3 oranges, the ratio of apples to oranges is always 2:3, regardless of how many fruits you have. In mathematical terms, if you have two variables, x and y, they are proportional if you can express them as y = kx, where k is a constant. This concept is often used in real-life situations, such as calculating speed or cost, where one quantity directly affects the other.