Typically 3/4 inch thick hardwood (oak, maple, cherry) really is 3/4 inch thick. This applies only to solid hardwood. If you buy 3/4 inch hardwood veneered plywood, it could be 3/4 inch, it could be 25/32 inch, or it could be something very close to 3/4 inch, but actually metric. In the case of plywood, you need to measure it very carefully.
In the case of engineered hardwood floors, I am not certain what the nominal thickness actually means, if you get engineered flooring you need to measure the actual thickness of the specific wood you are buying, as some of it seems to be metric.