Constructivism is a theoretical perspective in International Relations that argues that international relations is not solely based on material interests and power, but on ideas and norms that shape state and non-state actors' behavior and identity.
In his book, Alexander Wendt defines constructivism as follows: "Constructivism is a theory of knowledge according to which what we know is shaped by the meanings that actors give to things, and these meanings are in turn shaped by social practices and institutions."
Wendt, A. (1999). Social theory of international politics. Cambridge University Press.