Answer: A plane
Explanation:
In geometry, there are three undefined geometric figures that are used to define other terms in geometry.
i) A point ii) a line iii) a plane
A plane is a flat surface with two dimensions ( Length and width ) .
A line is a one dimensional geometric figure which extends upto infinity in both sides.
Parallel lines are the lines in plane which do not meet at any point.
Thus, the undefined term can contain parallel lines = A plane