Evidence is relevant if it is both probative and material. FRE 401.
Evidence is probative if it has any tendency to make a fact more or less probable than it would be without the evidence. FRE 401(a).
it has any tendency to make a fact more or less probable than it would be without the evidence
Evidence is material if the fact [that it is probative of] is of consequence in determining the action. FRE 401(b).
the fact [that it is probative of] is of consequence in determining the action
The credibility of every witness is at issue, and therefore, impeachment is always relevant, although not always allowed.
If evidence's relevance is conditional on a fact, admissible evidence must be sufficient such that a reasonable person could believe that the fact more likely than not does exist. FRE 104(b).