What exactly do we mean by access revocation?

The topic of access revocation tends to be a confusing one in the context of encryption and data sharing. In our presentations, when we mention the ease of access revocation with proxy re-encryption as one of our sources of value, attendees look on in slight bewilderment - How can you revoke access to data already shared?

This is a companion discussion topic for the original entry at https://blog.nucypher.com/what-exactly-do-we-mean-by-access-revocation/