A paradigmatic relation is a relation that holds between elements of the same category, i.e. elements that can be substituted for each other. It contrasts with syntagmatic relation, which applies to relations holding between elements that are combined with each other. The opposition between 'paradigmatic' and 'syntagmatic' relations is an important dichotomy of structuralist linguistics.
The term 'paradigmatic relation' was introduced by Louis Hjelmslev. Ferdinand de Saussure, who established the opposition between the two types of relations in structuralist linguistics, used the term associative relation for what Hjelmslev called paradigmatic relation.
Lyons, John. 1968. Introduction to Theoretical Linguistics. Cambridge: Cambridge University Press.