Which mathematical term cannot be defined for all cases in a system of geometry? parallel lines midpoint of a line segment distance along an arc diameter of a circle

The answers are :

1. parallel lines

2. midpoint of a line segment

Step-by-step explanation:

The mathematical terms that cannot be defined for all cases in a system of geometry are:

1. parallel lines and  

2. midpoint of a line segment.

A parallel line cannot be defined as lines do not occupy any space. It is a straight one dimensional figure with no thickness and it extends infinitely in both directions.

The points also do not have any dimension like length, area, volume etc.

fist on the left goes with the third on the right. second on the left goes with the fourth on the right. third on the left goes with the second on the right. the fourth on the left goes with first on the right