Two quantities are said to be proportional or directly proportional if their measurements evolve in the same direction.
In other words, two quantities are proportional if we can calculate the measurement of one by multiplying (or dividing) the measurement of the other by the same number. This number is called the coefficient of proportionality of these two quantities.
Example
The cost \(C\) of purchasing gas is directly proportional to the number \(L\) of litres purchased.