Question

A line is an undefined term because it

Answer 1

It is undefined because it is one of the building blocks of geometry.
In order to define a term, we must have a definition of all of the words used to describe it. As this progresses, we would have a never-ending list of words to define; at some point, we must accept a few basic building blocks as being undefined.

Answer 2

Answer:    

Step-by-step explanation:

A line is open from both sides from left hand side and right hand side.

It can be extended from both the sides which will lead it toward either $-\infty\text{or}\infty$ which is why it is said to be undefined.

If we take general equation of a line

y=m x+c

Its domain is all real numbers and its range is also all real numbers.

Real numbers can also be represented as $(-\infty,\infty)$.This is another way of writing the entire real number line.