A mirror is also a word for another server holding or sending a copy of media. If you type in 'google.com', do you think you go to Google.com? No, you're sent to one of the myriad mirrors of google.com that's nearest you. This let data be more resilient to network error and faster to access from various locations.
It's come in and out of common usage. Email lists were originally called 'mirrors' because the server reflected emails to a list of users, for instance. But as more and more of this technology becomes invisible, it'll be less likely for people to know it.
SP