Serene Highness is a style used to address or refer to some members of royal families. It is also used by sovereign, i.e. reigning, princes, such as the sovereign princes of Monaco and Liechtenstein.
When used as a direct form of address, spoken or written, it takes the form Your Serene Highness. When used as a third-person reference, it is gender-specific (His Serene Highness or Her Serene Highness, both abbreviated HSH) and, in plural, Their Serene Highnesses (TSH).