Which word best describes a situation that is harmonious?

The term "synergy" best describes a situation that is harmonious because it implies a cooperative interaction where different elements work together effectively to produce a combined effect greater than the sum of their separate effects. In a harmonious situation, individuals or components contribute positively and complement one another, creating an overall atmosphere of unity and collaboration.

This contrasts sharply with terms like conflict, tension, and disagreement, which all signify discord or opposition, indicating a lack of harmony rather than the cooperative spirit that synergy encapsulates. Thus, "synergy" is the most fitting choice when describing a state of harmony.