selector

A selector is a term used in computing to denote a construct that identifies a subset of items within a data structure, such as a document, a database, or a collection. It specifies criteria to filter or locate elements that match those criteria.

In web development, CSS selectors determine which HTML elements receive a given style. Simple selectors target by element name, class, or id; more complex ones combine criteria with descendant, child, adjacent, and general sibling relationships, and can target attributes, states (pseudo-classes) and structural patterns.

In the DOM and JavaScript, selector APIs such as document.querySelector and document.querySelectorAll retrieve a single element or a list based on CSS selectors. XPath provides a separate path-based selector language used with XML and some HTML contexts.

Outside the browser, selectors appear in databases for row filtering, in user interfaces for option picking, and in programming patterns that filter collections or stream data.

Key related concepts include specificity, the effect of order, and performance considerations when selectors are broad or complex.