Evidence for a given theory is the observation of an event that is more likely to occur if the theory is true than if it is false. (The event would be evidence against the theory if it is less likely if the theory is true.)

The likelihood ratio term from Bayes' theorem is greater than 1 if the event B is evidence of the theory A, and less than 1 if the event is evidence against the theory.

Blog posts

What is Evidence?
How Much Evidence Does It Take?
Scientific Evidence, Legal Evidence, Rational Evidence
The Lens That Sees Its Flaws
No One Can Exempt You From Rationality's Laws
That Alien Message
Information theory and the symmetry of updating beliefs by Academian

See also

Rational evidence, Evidence of absence, Extraordinary evidence
Filtered evidence
Generalization from fictional evidence
Conservation of expected evidence