X.509 certificate
From Just Solve the File Format Problem
X.509 certificate is an abstract certificate format used with X.509 public key encryption systems.
Certificates supply the public key of somebody you wish to communicate with securely, such as a client or vendor. They are used in electronic transaction processing, for instance. You may need to install a vendor's certificate file on your system in order to send transactions such as credit card sales to be processed.
An X.509 certificate may be stored in a number of different file formats, such as:
