What is a tissue?

When my husband was a little boy, he hurt his mouth enough times that he damaged one of his front teeth, and ever since then he wears a fake one, on a flipper.  (A retainer type thing that he can remove when he is eating.)  However the flipper breaks every few years, and then he is without a tooth until we can get it fixed, or a new one made.  He finally got tired of this and decided to get an implant.

The first step to getting this implant was to get a bone graft.  Which meant he needed to have an oral surgery.  After this surgery, one of the words that he would use often, when talking about it was gum tissue.  

Well one day, Guy asked me, "What is a tissue?"  Not thinking about my husband's recent surgery I responded with "Something you use to blow your nose."  

He then replied, "why do you have tissue in your mouth?"
"You don't have tissue in your mouth."
"yes you do.  Gum tissue."