A person or service designated to receive official legal documents and government notices on behalf of your business.
Every LLC and corporation is required by law to maintain a registered agent in the state where it's formed. The registered agent is a person or company with a physical street address (not a P.O. box) in that state, available during business hours, who can receive legal documents — lawsuits, government notices, tax forms — on behalf of the business.
If someone sues your business, the lawsuit papers (called service of process) must be delivered to your registered agent. If your registered agent misses a lawsuit notice, you might not find out about it until a default judgment has been entered against you. This is why using a professional registered agent service rather than putting your own home address is generally a good idea.
For Delaware companies operating in other states, you'll need a registered agent in Delaware (where you're incorporated) and typically also in your home state (where you're "foreign qualified" to do business). Professional registered agent services typically cost $50–$300 per year.