Question 776503
That should be the definition of midpoint.
I would define midpoint of segment AD as a point C, belonging to segment AD, such that segments AC and CD are congruent.
That sounds "official" enough.
Saying that it is the point that is exactly in the middle, or that it is at the same distance from both ends of the segment may give you a clearer idea of what midpoint is, neither is a rigurous definition.