Ok, possibly anti-style, but what does it mean when clothes are described as "thermal"

[isit]

I know it means they're supposed to be warm, but what exactly makes them thermal?

DSs have thermal vets, which certainly seem to keep them warm, but they're 100% cotton. What makes them different from other vests?

perhaps they are a tighter knit fabric so insulate better.

If you look on the damart website & I think M&S they actually score the "thermalness" of their thermal stuff