Definition: The support for a particular assertion, such as the existence of an interaction or pathway. Usage: At least one of confidence, evidenceCode, or experimentalForm must be instantiated when creating an evidence instance. XREF may reference a publication describing the experimental evidence using a publicationXref or may store a description of the experiment in an experimental description database using a unificationXref (if the referenced experiment is the same) or relationshipXref (if it is not identical, but similar in some way e.g. similar in protocol). Evidence is meant to provide more information than just an xref to the source paper. Examples: A description of a molecular binding assay that was used to detect a protein-protein interaction.