Originally Posted by Doohickie

Can a Hybrid have an internally geared hub?

My opinion leans toward no, but is not hardover that way.

Can a Hybrid have 26" (559mm) wheels as on most MTBs?

Certainly.

What differentiates it from a Touring bike other than handlebars and maybe a few additional braze-ons on the touring bike frame and fork for racks?

Not sure. I'm not familiar with what makes a touring bike a touring bike to offer an opinion.

My definition of a hybrid is as follows:

Start with a hardtail mountain bike configuration. Add a fixed suspension fork. Lose the knobby tires and add something with less rolling resistance. Maybe had some mounting lugs and braze-ons for racks, fenders & water bottles. Flat handlebar can remain, or other configurations may be used (various other upright bars, trekking bars, or even drop bars) according to use.

OR

Start from a road bike, preferably with a steel frame. Add wider tires to make them more forgiving on rough surfaces. Add a MTB-geared triple chain ring. Add more room for fenders and add lugs & braze-ons. Change out handlebars to suit (as above).

I think the point of a hybrid is to have a fairly rugged bike (steel frame, wide tires) that is not necessarily weight optimized but does have fixed suspension. To me a hybrid bike is optimized for the variety of surfaces found in suburban and urban riding, while still trying to maintain speeds near to that of a road bike.