The person or company a business entity must designate to receive lawsuits and official notices on its behalf - and the address at which a default judgment can quietly begin.
Every corporation, LLC and similar entity must name a registered agent in the state where it is formed, and in each additional state where it registers to do business. The agent is the entity's designated recipient for service of process - the papers that start a lawsuit - and for official correspondence from the state. The agent must have a physical street address in the state and be available during business hours; a post-office box does not qualify.
The role sounds clerical and its failure is not. If the agent moves, resigns, or is a founder who long ago stopped checking that address, a lawsuit can be validly served on an entity that never learns of it, and a default judgment can be entered against a company that would have had a complete defence. Failing to maintain an agent is also a ground on which the state can revoke the entity's good standing or administratively dissolve it, which can in turn put the owners' liability protection at risk.
Any adult resident of the state, or a company authorised to act as an agent there, can serve. Businesses commonly use a commercial registered-agent service for a fee so that the address never changes when the business moves and someone is always there to sign.
This is rarely something to hire a lawyer for on its own, but it is the first thing a lawyer will check when a business is sued or discovers a judgment it never saw coming. The practical advice is simpler than legal advice: know who your agent is in every state where you are registered, and make sure that person or service knows how to reach you today.
Choose your state. Each link opens the directory page for the city in that state with the most currently published law firms in this practice area; a +n beside the city is how many other cities in the state also have one. The list is generated when this page loads, so a state whose listings have lapsed drops out rather than becoming a dead link.