Locus (plural loci) is a Latin-derived term meaning "place" or "location." Across disciplines it names either a specific point or a set of points that share a defining property. Because of its literal meaning, the word appears in technical, scientific, legal and everyday language with related but distinct senses.
Mathematical meaning
In geometry and algebra, a locus is the collection of all points that satisfy a given condition. Typical examples include the set of points equidistant from two fixed points (the perpendicular bisector) or the set of points whose distance to a focus and a line have a fixed ratio (conic sections). In analytic geometry a locus is often described by an equation; in algebraic geometry it corresponds to solution sets of polynomial equations. The notion also appears in dynamical contexts, where a locus can be the trace or path of a moving point under varying parameters.
Genetics and biology
In genetics a locus denotes the specific location of a gene or genetic marker on a chromosome. Each gene occupies a locus; different versions of a gene (alleles) are said to occur at the same locus. The concept is central to genetic mapping, linkage analysis and the study of inherited traits: identifying a locus associated with a disease points to a neighborhood of the genome for further study.
Psychology and social science
The phrase "locus of control" refers to a psychological construct describing whether people perceive outcomes as the result of their own actions (internal locus) or external forces such as luck or fate (external locus). Introduced in mid‑20th century personality research, the concept helps explain differences in motivation, coping, and behavior in response to success and failure.
Other uses and notable facts
Beyond these fields, "locus" appears in medicine to indicate the site of a lesion, in legal Latin phrases (for example, locus standi meaning the right to take legal action), and as the title of periodicals and publications in arts and sciences. Because it is concise and evocative, the term is favored when a precise notion of place, origin or position is required.
Distinctions and usage tips: use "locus" when referring to a defined place or set with a technical meaning; use "site," "position" or "location" in more general contexts. Remember the plural is "loci," and in many disciplines the exact interpretation of a locus depends on the formal system in which it is defined.