Dimensionality is the combination of basic dimensions associated with a physically significant quantity. These dimensions are those of mass M, length L, and time T and any combinations of their powers. For instance, momentum has dimensions of mass × velocity (MLT -1) and energy has the dimensions of force × distance (ML2 T -2).
Equations describing physical quantities can be checked by ensuring they have the correct dimensions. Sometimes, it is possible to define quantities in which the dimensions cancel out. These dimensionless quantities are significant as they are entirely independent of the conventions used to define mass, length and time. For more on this topic, see dimensional analysis.