Wiktionary
n. a special type of mail that provides more control than regular mail.
WordNet
n. mail that is registered by the post office when sent in order to assure safe delivery [syn: registered post]
Wikipedia
Registered mail is a mail service offered by postal services in many countries, which allows the sender proof of mailing via a mailing receipt and, upon request, electronic verification that an article was delivered or that a delivery attempt was made. Depending on the country, additional services may also be available, such as a chain of custody, where the posted item has its details recorded in a register to enable its location to be tracked, sometimes with added insurance to cover loss. The name of this service varies from country to country, and postal services in some countries offer more than one level of service under different names (e.g., "certified mail" and "registered mail" in the U.S.). See the country-specific information below for details.
Usage examples of "registered mail".
I get a really limited selection of registered mail, and it's never good news.
The size of registered mail packages varies greatly, but 1000 packages for each mail bag should be a conservative guess.
He got himself hidden in the car somehow and up in Wyoming he shot the clerk, cleaned out the registered mail and dropped off.
He had the regular mail in one hand and one of those padded envelopes that come registered mail just about every week from New York―.