relativity

American  

[rel-uh-tiv-i-tee] / ˌrɛl əˈtɪv ɪ ti /

noun

1. the state or fact of being relative.

2. Physics. a theory, formulated essentially by Albert Einstein, that all motion must be defined relative to a frame of reference and that space and time are relative rather than absolute concepts: it consists of two principal parts. The theory dealing with uniform motion special theory of relativity, or special relativity is based on the two postulates that physical laws have the same mathematical form when expressed in any inertial system, and the velocity of light is independent of the motion of its source and will have the same value when measured by observers moving with constant velocity with respect to each other. Derivable from these postulates are the conclusions that there can be no motion at a speed greater than that of light in a vacuum, mass increases as velocity increases, mass and energy are equivalent, and time is dependent on the relative motion of an observer measuring the time. The theory dealing with gravity general theory of relativity, or general relativity is based on the postulate that the local effects of a gravitational field and of acceleration of an inertial system are identical.

3. dependence of a mental state or process upon the nature of the human mind.

   relativity of values; relativity of knowledge.

---

relativity British  

/ ˌrɛləˈtɪvɪtɪ /

noun

1. either of two theories developed by Albert Einstein, the special theory of relativity , which requires that the laws of physics shall be the same as seen by any two different observers in uniform relative motion, and the general theory of relativity which considers observers with relative acceleration and leads to a theory of gravitation

2. philosophy dependence upon some variable factor such as the psychological, social, or environmental context See relativism

3. the state or quality of being relative

---

relativity Scientific  

/ rĕl′ə-tĭv′ĭ-tē /

1. Either of two theories in physics developed by Albert Einstein, General Relativity or Special Relativity.

2. See Notes at Einstein gravity space-time

---

Closer Look

Albert Einstein's two theories of relativity were the first successful revisions of Newtonian mechanics—a mechanics so simple and intuitive that it was held to be a permanent fixture of physics. Uniting the theories is the idea that two observers traveling relative to each other may have different perceptions of time and space, yet the laws of nature are still uniform, and certain properties always remain invariant. Einstein developed the first theory, the theory of Special Relativity (1905), to explain and extend certain consequences of Maxwell's equations describing electromagnetism, in particular, addressing a puzzle surrounding the speed of light in a vacuum, which was predicted always to be the same, whether the light source is stationary or moving. Special Relativity considers the laws of nature from the point of view of frames of reference upon which no forces are acting, and describes the way time, distance, mass, and energy must be perceived by observers who are in uniform motion relative to each other if the speed of light must always turn out the same for all observers. Two implications of Special Relativity are space and time dilation. As speed increases, space is compressed in the direction of the motion, and time slows down. A famous example is the space traveler who returns to Earth younger than his Earth-dwelling twin, his biological processes proceeding more slowly due to his relative speed. These effects are very small at the speeds we normally experience but become significant at speeds approaching the speed of light (known as relativistic speeds). Perhaps the best-known implication of Special Relativity is the equation E=mc², which expresses a close relation between energy and mass. The speed of light is a large number (about 300,000 km per second, or 186,000 mi per second), so the equation suggests that even small amounts of mass can be converted into enormous amounts of energy, a fact exploited by atomic power and weaponry. Einstein's General Theory of relativity extended his Special Theory to include non-inertial reference frames, frames acted on by forces and undergoing acceleration, as in cases involving gravity. The General Theory revolutionized the way gravity, too, was understood. Since Einstein, gravity is seen as a curvature in space-time itself.

Other Word Forms

• nonrelativity
  noun

Etymology

Origin of relativity

First recorded in 1825–35; relative + -ity

Explanation

Relativity is the word for how things only have importance in relation to other things. In physics, relativity refers to Einstein's theory that time and space are not absolute. If you believe there are absolute, correct answers to everything, you probably don't like relativity, which is the opposite of absolutism. According to relativity, things aren't good or bad in themselves; they only have significance in relation to other things. In physics, relativity is about how even time and space aren't absolute, objective facts. For example, time partially depends on our perceptions of it: time is relative.

Example Sentences

She said she would be watching from the sidelines when the society achieved its aims as she had accepted a job with US aerospace manufacturing company Relativity Space, which would begin after her graduation.

From BBC • Apr. 13, 2026

"If confirmed, it could help us better understand both our own Galaxy, and General Relativity as a whole."

From Science Daily • Feb. 17, 2026

Stoke Space plans to launch an expendable rocket next year to collect data for its push for reusable vehicles, while Relativity Space wants to test its medium-to-heavy lift, reusable rocket in late 2026.

From MarketWatch • Dec. 30, 2025

Mr. Schmidt is chairman of Relativity Space and a former CEO of Google.

From The Wall Street Journal • Nov. 18, 2025

So I went on my zigzag way across the common, thinking about time, black holes, Einstein’s Theory of Relativity and storm warnings.

From "The London Eye Mystery" by Siobhan Dowd

Related Words

• accord
• affair
• communication
• contact
• exchange
• liaison
• link
• marriage
• rapport
• relation
• tie