The tag was on the question "Photoisomerization of Azobenzene", see the edit history here.
I am responsible for making this tag an orphan, meaning as soon as I saw it, I substituted it to the proper tag. I do things like this at least once a week and I outlined the procedure somewhere on meta before. Unused, they are called orphaned, tags will be removed from the register. And if you go to the tags page, you won't be able to find it any more. Unfortunately I cannot remember where it says, when they are cleared, but I think it was 03:00 UTC. The help centre does not mention it, maybe the faq would - and it does: How can we get rid of misspelled and unused (or "zombie") tags?
I remember, that this was not the first time I burned that particular tag and I am very much pro banning it. It's just like fighting the title buzzwords, you have to keep hunting them down. I admit it's a lot easier with tags.