The word uniform means the "same in all cases". Therefore, constant and uniform are often used interchangeably. Just be careful about where and when the sentence is used. For instance, if I say a car is accelerating uniformly in the xx-direction, then it has a constant acceleration in that direction.
In the case of a car moving around a circle with a constant speed, then we have uniform circular motion, but the velocity in this case is not uniform, it is changing direction. The word "uniform" in this case means it is travelling at a constant speed, and in such a case the magnitude of velocity is constant, but the direction changes.
Then, constant is with respect to time domain and uniform is said to be with respect to the space domain.
Now, consider a metallic cube. Now we say that mass is uniform if the mass per unit volume is same everywhere.
Then, what is constant?
Consider the same example. Now we say that the mass is constant if the mass of the whole cube does not change with respect to time.
P.S.: I went through J. Redman's answer and interpreted it in a lucid way. That is why the first two paragraphs of this answer resemble J. Redman's answer.