The tag can not stand alone. Zend is a company. A VM with their name is what powers PHP (zend-engine). They have a PHP certification program (zend-certification). They make an IDE (zend-studio), a web server (zend-server, zend-server-ce), a debugger (zend-debugger) and a bytecode encryption package (zend-guard). They've also leant their name to the Zend Framework (zend-framework).
But there is nothing that would be on topic on SO that could possibly be called Zend and Zend alone.
The vast majority of the questions tagged zend are going to be for Zend Framework. 70 of them are co-tagged with it, in fact. Argh!
While I am going to spend a few days editing all of the ~230 questions into their proper tags, I would really like to get a synonym put in place or a blacklist of the tag. To be clear, the synonym/blacklist shouldn't be done until the questions are already in their proper place, as not all of the currently mis-tagged questions belong in zend-framework.
I have now gone through all of the questions tagged zend and updated the tags on those not about Zend Framework. There were about 25. There are a few that are about multiple Zend things simultaneously, I've left those alone when one of those things will end up being Zend Framework. There were about 15 that were tagged zend+frameworks ("frameworks" having a synonym of "framework") that I've corrected.
Overall, the tagging quality on these questions is pretty abyssmal, but considering that the askers didn't pay enough attention to notice that they were using the wrong tag to begin with, I'm entirely unsurprised.