A construction is a conventional schema for creating or motivating well-formed expressions in which there is at least one open slot that can be filled by one of several expressions belonging to the same form-class.
The definition here is from Haspelmath (2023). It is different from widespread definitions adopted by Construction Grammar representative, where not only schemas, but also forms (including words) are regarded as constructions. However, these definitions are rarely applied in practice, so the definition given here actually accords well with the practice of Construction Grammar.
construction = the basic unit of morphosyntactic analysis; a construction is a conventional pairing of form and function – its form is morphosyntactic structure, and its function is a combination of meaning (semantic content) and information packaging (Section 1.1). When combined with a modifier describing a specific construction, ‘[Modifier] construction’ refers to any pairing of form and function in a language (or any language) used to express a particular combination of semantic content and information packaging denoted by the modifier of ‘construction’ (Section 1.4). Example: the numeral modification construction exemplified by three tree-s consists of a form which: (i) can be described schematically as [NUM NOUN-NUMBER]; (ii) performs the function of referring to a group of objects of the type denoted by the noun (tree), and modifying that information with the additional information that the cardinality of the group is the amount denoted by the number (three). Specific constructions (a.k.a. cri- teria, tests, evidence) are used to define word classes. (Section 1.2.3)