What does colinear mean?
Colinear is an adjective used almost entirely in mathematics and the sciences to describe points, objects, or quantities that lie on, or act along, one and the same straight line. When three or more points fall on a single line, they are said to be colinear — a property easily tested by checking whether the slopes between successive pairs of points match. The term also extends to vectors and forces whose lines of action coincide or run parallel, making them behave as if confined to one axis. It carries a formal, technical tone and appears rarely outside textbooks, papers, and technical reports. A closely related spelling, "collinear," is equally acceptable and often preferred in modern usage, so readers should not treat the difference as a change in meaning. Colinear contrasts naturally with coplanar, which concerns shared planes rather than shared lines. Understanding the word is useful for anyone working with coordinate geometry, physics problems involving force diagrams, or data analysis where alignment of points matters.
adjectiveLying on, or passing through, the same straight line. Used chiefly in mathematics to describe points that share a single common line.
- (of points) lying on one and the same straight line
- (of vectors or forces) acting along the same line or along parallel lines sharing a common direction
"The three points proved colinear once their coordinates were plotted on the graph."
"Because the three towns sat colinear along the old railway route, the surveyors could lay the new road in a single straight stretch."
"In vector analysis, colinear forces simplify the problem because their components can be added algebraically along one axis."
Any two points are automatically colinear — but the moment you add a third, there's a real chance they refuse to line up.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of colinear
Colinear comes from Latin, combining the prefix co- (meaning "together") with linea, meaning "line." English borrowed linea through words such as "line" and "linear," which entered the language from Old French in the medieval period, with "colinear" itself emerging much later as scientific vocabulary. It shares its root with related terms like "lineal," "lineage," and "delineate," all tracing back to the same Latin source for thread-like straightness.
Related word forms
How colinear is actually used
Formal and technical, appearing almost exclusively in mathematical and scientific writing. The double-L spelling "collinear" is at least as common in modern usage, and some style guides prefer it; both spellings denote the same concept.
Easily confused with colinear
"Collinear" is merely the more common spelling variant of "colinear"; both mean lying on the same line, though dictionaries often list only "collinear."
"Coplanar" describes objects lying in the same plane (a flat surface), while "colinear" describes them lying on the same single line.