Parallel Lines
Parallel lines are two or more coplanar lines that never intersect, no matter how far they are extended in either direction. They always maintain the exact same distance apart.
Example
In a coordinate plane, two non-vertical lines are parallel if and only if they have the same gradient (slope), $m_1 = m_2$. For instance, the lines $y = 2x + 3$ and $y = 2x - 5$ are parallel.