The term "noninstitutional" refers to something that is not associated with any formal organization or institution. Some potential antonyms for this term could include "institutional," "formal," "official," "established," or "organized." These words suggest a sense of structure, control, and regulation, which contrasts with the idea of being noninstitutional. Other antonyms might include "informal," "unstructured," "unregulated," "undocumented," or "ad hoc." These terms imply a more spontaneous or improvised approach to a given situation, without the constraints or requirements that come with being part of an institution or formal organization.