Multi-Domain SSL certificates also known as UCC (Unified Communication Certificates) or SAN (Subject Alternative Name) cartificates.
If you have registered several domains and would like to secure all of them with just a single certificate then Multi-Domain SSL is exactly what you are looking for.
These certificates secure several domains/sub-domains with just one certificate by using same Private Key and CSR. This simplify a lot SSL management process on your side by managing up to 250 domains/sub-domains within a single certificate. You can always reissue existing certificate by adding additional domain(read SAN) to extend your security coverage, of course this mean that you will need to reinstall new certificate in all previously installed places.
SAN/UCC certificates are compatible with Microsoft Exchange and Microsoft office communication server.