Dictionary.net

An attachment to an electronicmail message used for security purposes, e.g. to verify thata user sending a message is who he or she claims to be, and toprovide the receiver with the means to encode a reply.An individual wishing to send an encrypted message applies fora digital certificate from a certificate authority (CA).The CA issues an encrypted digital certificate containing theapplicant's public key and a variety of other identificationinformation. The CA makes its own public key readilyavailable on the Internet.The recipient of an encrypted message uses the CA's public keyto decode the digital certificate attached to the message,verifies it as issued by the CA and then obtains the sender'spublic key and identification information held within thecertificate.