Answer : The ratio of the mass of a substance to its volume is called density.
Explanation :
Density : It is defined as the mass of a substance contained per unit volume.
The unit of mass is gram (g) and the unit of volume is milliliter (ml). So, the unit of density is g/ml.
Formula used :

For example : If mass of object is 550 grams and volume of water displaced 25 ml. Calculate the density of an object.
By using formula, we get:

Thus, density of the object will be 22 g/ml