k guys need help and only have 40 min pls help. description of an undefined term?

The distance along a line segment is the number of units between two points.
Perpendicular lines are lines that intersect at a right angle.
A ray extends infinitely in one direction from an endpoint.
An angle is the figure formed by the intersection of two rays at a common endpoint.
2.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

Question

k guys  need help and only have 40 min pls help. description of an undefined term?

The distance along a line segment is the number of units between two points.
Perpendicular lines are lines that intersect at a right angle.
A ray extends infinitely in one direction from an endpoint. 
An angle is the figure formed by the intersection of two rays at a common endpoint.
2.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