2 servers can do "Mirroring" are there 3 servers in Sharepoint 2010 Server mirroring?

Hello Community

    In providing availability using Sharepoint 2010 Server using "Mirroring"
you need a "Principal, Mirror and a Witness".

    Logically speaking the principal and mirror I would say are the two servers
that are involved in Sharepoint 2010 Server availability.

    The question is in a 2 server farm what is the "Witness" in availability,is it a Sharepoint
2010 Server, is it the Domain Controller or is it a separate server and do you have
to purchase a copy of Sharepoint 2010 Server to run it on?

    Thank you
    Shabeaut