Q:

Difference between "magnitude" and "multitude"

What is the difference between magnitude and multitude?

Quantity is a kind of property which exists as magnitude or multitude.

A:

Magnitude 
Definition: Extent of dimensions; size; applied to things that have length, breath, and thickness.                                                             
Multitude
Definition : The state of being many; numerousness.
For more details you can follow these links -
Link 1 : Magnitude
Link 2 : Multitude
Hope it'll help :-)

