Not your everyday true solid hardwood. Junckers makes a clip system where you can float a solid

Here is a link that might be useful: Clip System

Why would I need a specific clip system just because it's hardwood? Couldn't I glue the hardwood just like any other floor (that is, glue the pieces to each other, not glue them to the floor)?

It seems some places I look they say that you can float a hardwood, but when I go to sites that sell hardwood online, when I say I want floating, it directs me to engineered.

engineered is real wood, your confusing terms and language. Engineered can come in individual strips 3 or 4 inches thick, or come as a plank (like a laminate) with 2 or 3 'strips' of hardwood on each plank. Floating floors are generally engineered cause they are far more stable than a solid wood and won't expand or contract as much. The tongue and groove system is different on floating floors than a solid. I think you need to talk to someone who can show you a variety of wood samples etc. You can get engineered in a sliced veneer that would be indistinguishable from a solid.

It is much better to use an engineered product for a radiant system although I have never heard of an install over an electric system. Solid wood is never straight enough to glue except maybe the Junckers solid as mentioned.
Basically you can not float a solid floor and it is possible that you can't even install engineered over electric.

You may want to visit the Carlisle Wood Flooring website linked below. I'm ordering their solid wood floor to glue down on a concrete slab, and I believe their wood can also be used over radiant heat. I understand this is because they only use old-growth wood (at least 200 years old) and also cure the wood over a one year period, so there's no moisture left. (Sorry if I'm not using the correct terms - just a consumer here.)

I've always loved their wide-plank floors that I've seen in their advertisements, but assumed I couldn't use the product because we're on a slab, so I'm really happy that I did my research here on GW and found out this isn't the case. So far, I've been extremely happy with the great customer service and knowledge of my sales rep, and they'll send you tons of information and floor samples to help you make your choice. The website is also very informative.

Oh - and the floors are gorgeous! They offer old-growth and antique walnut, oak, chestnut, pine, etc. They're based in New Hampshire but I visited their new showroom in California because I had to see the product in person before ordering.

We went with the intention of ordering white oak or walnut but fell in love with the wide-planked (up to 20" wide and 16' long) white eastern pine. We ordered 13" to 17" and they're throwing in enough 20" wide planks to give us a few runs at no extra cost. Cost is $8 sf - the same as the engineered merbau used in my dd's room. It starts at $6 sf for thinner planks, and I believe pricing for other woods is listed on the website. We're also ordering an antique beam from them to be used as a mantle for our fireplace, and I believe it's $3-400 for one 7ft long.

Here is a link that might be useful: Carlisle Wood Floors

There is always moisture in wood. Moisture content can be brought up or down some by the drying process, but it cannot be reduced to 0%.

More solid products are being adhered directly to tested and properly prepared concrete substrates or to a moisture vapor retarder.

There are now special adhesives available to glue down solid products. This must be done by someone who knows how to do it.