The direct relationship between temperature and volume is known as

Charles' law

Explanation:

Charles' law is one of the Gas Laws. Gas laws were given in 18th century and they refer to the relationship between temperature, pressure and volume of gases. Charles' law, given in 1787 by Jacques Charles, states that the volume and temperature (in Kelvin) of a sample of gas is directly proportional to each other provided it is an ideal gas and the pressure is constant.

Lets say,

Volume = V and temperature = T, then

V∝T