Direct and inverse proportion refer to the relation between two variables that defines how the value of one variable changes when the value of the other variable is changed.
In case a variable y is directly proportional to x, when x increases y increases and when x decreases y decreases.
On the other hand if y is inversely proportional to x, when x increases y decreases and when x decreases y increases.
The term direct proportion is used when the change in the values of two related variables has the same sign. Inverse proportion is used when the change in the value of two related variables has an opposite sign.