Glossary
Relevance
The basic test for whether evidence is admissible: does it tend to make a fact that matters to the case more or less likely? Irrelevant evidence is excluded.
Relevance is the floor for admissibility. Even if evidence is relevant, it can still be excluded for other reasons: unfair prejudice, confusion, waste of time, hearsay, privilege, etc. But evidence that isn't relevant can never come in.
The standard is broader than people think. Evidence doesn't have to be conclusive or even strong. As long as it makes a relevant fact "more or less probable than it would be without the evidence," it clears the relevance bar.