** Parallel** has four important meanings:

- A line of latitude, so called because pairs of latitude lines are a fixed distance apart and appear superficially to be parallel, whereas longitude lines do not. (in fact, the converse is true: latitude lines are concentric circles not straight lines, and longitude lines are great circles, that is straight lines in the local non-Euclidean geometry. Lines of latitude are often used as arbitrary boundaries between countries or regions: examples include the 38th, 42nd, 45th, 49th, and 60th parallels.
- In electricity and electronics, components are said to be "in parallel" if current branches to flow through both simultaneously, as against "in series" where the current flows through first one, then the other, along a single path.
- In telecommunications and computing, data transfer is parallel if there are several data lines (typically one for each bit of a word), and at each clock pulse a bit simultaneously travels down each line; in serial communications there is only one line, and only one bit goes at a time.
- A property of straight lines, described below:

** Parallel** is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. The existence and properties of parallel lines are the basis of Euclid's parallel postulate.

When lines or planes are parallel, then every point on one is located exactly the same minimum distance from the other line or plane. Another way of defining it is that any two parallel lines or planes, if extended to infinity in both directions, will never intersect. This second definition carries the condition that the extension must occur only in one additional dimension: In other words, parallel lines must be located in the same plane, and parallel planes must be located in the same three-dimensional space. A parallel combination of a line and a plane may be located in the same three-dimensional space. A third definition is if two lines are both intersected by a third line in the same plane, and the angles of intersection are equal, then the two lines are parallel.