onesided

One-sided, sometimes written as onesided, is an adjective describing something that involves or affects only one side, or that favors one position over another. In everyday language it can describe unfair or biased situations, unilateral actions, or disputes and contests that are not symmetrical.

In geometry and topology, a surface or object is described as one-sided if it lacks a distinct pair of sides. The Möbius strip is the classic example: a surface with only one boundary and one continuous side. Non-orientable surfaces, such as the Möbius strip and the Klein bottle, do not admit a consistent choice of “left” and “right,” and are often described as one-sided or having only one side. By contrast, orientable surfaces, like the sphere or torus, are two-sided.

In statistics, a one-sided (or one-tailed) test assesses whether a parameter lies in a specified direction away from a null value, rather than merely differing from it. A one-sided test has a rejection region in only one tail of the distribution, which affects p-values and required sample size. One-sided tests are appropriate when prior theory or evidence establishes the expected direction of an effect.

In common usage, one-sided can describe biases in reporting, negotiations, or decision-making that favor one party or outcome and do not address the other side. The term thus spans mathematical, scientific, and practical contexts, reflecting directionality or asymmetry in its various applications.