Definition of Midpoint

If you have gotten to algebraic proofs, then this is for you. To help demonstrate this, we have a diagram:

Source^

The two hash marks, one being between A and B, the other being between B and C, just show us that those two segments are congruent. The Definition of Midpoint states that:

                                   ___

If B is the Midpoint of  AC then

___   ___ 

AB = BC

Pretty much it's saying that If you are given the midpoint, 'B', then it's safe to say that one half of the line segment, 'AB', equals the other, 'BC'.