Effective date
The date a final rule takes legal effect.
What it is
The effective date of a final rule is the date on which the rule becomes law. From the effective date forward, the rule is part of the CFR and is enforceable by the issuing agency.
Plain-English explanation
When a final rule publishes in the Federal Register, it includes both an effective date and (often separately) a compliance date. The effective date is when the rule becomes law. The compliance date is when regulated parties must actually comply with the new requirements.
These can be different. A rule may be effective immediately and require compliance six months later, giving regulated parties time to implement.
Why it matters
Citing the wrong date in an audit response is the kind of finding nobody wants. Effective vs. compliance is the most common confusion in regulatory work.