Which undefined geometric term can be described as a one-dimensional set of points that has no beginning or end?

The three undefined terms is geometry are:
1- The point
2- The line (straight line) : one dimensional-figure
3- The plane :  two dimensional figure

Now, joining a set of points in a one-dimensional aspect forms a line. Since we are given that there is no start or end for this line, then it is a straight line.

Therefore:
Answer is: straight line