- Waiver = the intentional relinquishment or abandonment of a known right. Waiver requires an existing right, the waiving party's knowledge of that right, and an actual intention to relinquish that right. Waiver always rests on intent, which may be express or implied.
- Estoppel = generally requires a showing that a party's words or acts have induced detrimental reliance by the opposing party.
- Forfeiture = arises from a party's failure to preserve a claim by raising a timely objection.
See also Bryan Garner's latest On Words column here: Learn the fundamentals of writing first—experiment later