Parallel describes lines, paths, or things that run alongside each other at exactly the same distance apart, never touching and never getting closer or farther away. Think of railroad tracks stretching into the distance: they stay the same width apart for miles and miles. The lines on notebook paper are parallel. The opposite sides of a rectangle are parallel.

In geometry, parallel lines are marked with matching arrow symbols to show they'll never intersect, no matter how far you extend them. This is different from lines that cross (called intersecting lines) or lines that seem parallel but actually meet eventually (called converging lines).

The word also describes things that are similar or comparable, happening at the same time or developing in similar ways. A teacher might draw a parallel between two historical events that unfolded in strikingly similar ways. Scientists might notice parallel evolution when unrelated animals develop similar features, like how bats and birds both evolved wings for flight. When two friends have parallel experiences, they're going through similar situations even though their lives are separate.

As a verb, to parallel something means to match or closely resemble it: “Her dedication to practice paralleled her teammate's commitment.”