Superconductivity can only be achieved at incredibly low temperatures (low to mid double digit Kelvin temperatures). High temperature superconductivity is superconductivity that can be achieved at or near room temperature.
The term HTS may also refer to a particular class of superconductors, which demonstrates high critical temperature (actually, there is only one) But this particular materials may be called HTS even if they have rather low critical temperature. They are called so to emphasize that they have a crystal structure and a superconducting mechanism similar to their relatives. These materials are usually very poor conductors in their nonsuperconducting state and very brittle. Their mechanical characteristics are similar to those of ceramics and they have a layered crystal structure. Sometimes they are called "bad" metals.
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