A line is a one-dimensional object of infinite length with no width or thickness that never bends or turns. Its graphical representation is a straight line with arrowheads on either end, indicating that it continues indefinitely in both directions.

Through any two different points there is exactly one line. A line can be named using any two points on it. The above line could be named $\overleftrightarrow{AB},$ $\overleftrightarrow{BA},$ or line $\ell .$ When two or more points lie on the same line, they are said to be collinear.