Relevant evidence has any tendency to make a fact of consequence more or less probable. The threshold is deliberately low — a brick is not a wall.
Fact of consequence. Determined by the substantive law and the pleadings. Evidence bearing on an element, a defence, credibility, or damages qualifies.
The balancing test. Relevant evidence may be excluded if its probative value is substantially outweighed by a danger of unfair prejudice, confusing the issues, misleading the jury, undue delay, wasting time, or needlessly presenting cumulative evidence.
Unfair prejudice means an undue tendency to suggest a decision on an improper basis — usually an emotional one. All damaging evidence is prejudicial; only some of it is unfairly so.
Substantially outweighed puts the thumb on the scale toward admission, and the balance is reviewed for abuse of discretion.
Limiting instructions are the usual middle course: evidence admitted for one purpose with the jury told not to use it for another. Request the instruction at the time, and again in the final charge.
In commercial cases the recurring fights are over a party’s wealth, unrelated prior disputes, settlement discussions, subsequent changes to a practice, and inflammatory internal emails whose probative value is modest.