Which term describes something that has been observed or experienced?

The term that describes something that has been observed or experienced is "fact." In scientific terminology, a fact refers to a statement that can be objectively verified and is accepted as true based on empirical evidence. Facts are the foundation of scientific inquiry because they provide reliable data that can be measured or observed. For instance, the boiling point of water at standard atmospheric pressure is a fact; it can be consistently demonstrated through experimentation.

Other terms listed might relate to observations but do not fit the description as precisely. A theory is a well-substantiated explanation of an aspect of the natural world that is based on a body of evidence but is not simply an observation. A hypothesis is a proposed explanation made on the basis of limited evidence as a starting point for further investigation; it requires testing to determine its validity. An assumption is something taken for granted without proof, often used as a basis for further reasoning or discussions, which does not necessarily require observational support. Hence, "fact" is the most accurate term for something that has been observed or experienced.