Final answer:
A relationship where the ratio of one quantity to another remains constant is a proportional relationship, often referred to as direct proportionality, represented by the equation y = kx.
Step-by-step explanation:
A relationship in which the ratio of one quantity to another remains constant is called a proportional relationship. This is also known as direct proportionality. In such a relationship, when one quantity increases, the other quantity increases at a constant rate, and when one quantity decreases, the other quantity decreases at the same constant rate. This can be represented by the equation y = kx where k is the proportionality constant and both x and y are variables. A graph of a direct proportionality will show a straight line that passes through the origin (0, 0).