What scientific term is a well-tested explanation for a set of observations or experimental results?

The correct answer will be- A scientific theory.

Explanation:

A scientific theory is a substantiated idea which provides a deep explanation of any natural event.  A theory is an elaborate form generalized or proposed hypothesis which explains the phenomenon to a limited extent.

A theory is formed by combining many proved hypothesis which provides evidence as well as supports the hypothesis. The theory is accepted only after it has been proved by the experiments of the researcher and fellow scientists.

Thus, A scientific theory is the correct answer.