A derivational construction is a construction in which a meaning other than an inflectional meaning is expressed by an affix or endophonically.
The definition is adapted from Haspelmath (2024: 66). It defines derivational constructions as affixal (or endophonically marked) constructions where the affix does not denote an inflectional meaning. (Inflectional and derivational constructions are thus defined as kinds of functionally defined constructions, not as kinds of strategies.)
Note that "derivation" is often defined in terms of the notion of "lexeme", but it seems that the directionality must be different ("lexeme" must be defined in terms of "derivational construction").
"New lexemes that are formed with prefixes and suffixes on a base are often referred to as derived words, and the process by which they are formed as derivation." (Lieber 2009: 33)
"Derivation is part of the much wider phenomenon of lexical relatedness. It is often assumed that derivation takes a lexeme (the base lexeme) as input and delivers another lexeme (the derivate)." (Spencer 2015: 301)