Contrast is usually understood in photo terms to mean the rate of increase of density with increased exposure. Grades 0 to 5 on Ilford Multigrade paper all have the same maximum black density, for any given surface. High contrast is then achieved from a high rate of increase of density from small increases in exposure. Low contrast needs a much lower rate of density increase as exposure increases.

On film, contrast is usually called the Gamma and you choose the Gamma to develop your film to bearing in mind the paper you will print onto or the contrast needed for your final purpose, such as scanning, etc. Gamma is the slope of the straight line portion of the density against exposure sensitometric curve.