What is a postmark?

You might have noticed that I rarely uses this term to describe my material. We all refer to datestamps as postmarks. Actually, a datestamp is a type of postmark. Postmark is an all encompassing term to describe any markings on an item which has gone through the postal process. This would include datestamps, killer bar cancellations and also all the extraneous markings indicating routing, directory service, postage due, censorship, registration, special delivery, forwarding, carriage by special transportation and many others.
Datestamps are postmarks which show the date and sometimes the time of posting as well. Cancellations are those portions of the postmarks which marks(cancels) the adhesive to prevent it from reuse. A purist would also not describe an adhesive as a stamp until it has been cancelled (or stamped!) after passing through the normal postal process.
Therefore there are no such things as mint stamps and more usefully described as "adhesives".. It also follows that a CTO item can not be consider as a stamp either because it was purely manufactured and has never passed through the mailstream. A cancelled by favour item may or may not have passed through the mailstream which will make it difficult to be pedantic. Postage dues are sometimes described as "labels".
It is all very interesting and confusing. At the end of the day it does not really matter so long we derive pleasure from whatever we are collecting.