You are correct with Mayday and Pan however there are generally four categories of emergency.
2. Mayday Relay (Where you are not in emergency but have seen or heard someone who is and they have not been answered)
The ICAO definition of number 4 is :
Securite (/seɪˈkjʊərɨteɪ/; from French sécurité — safety) indicates a message about safety, such as a hazard to navigation or weather information.
Also I know it does not seem to apply to the USA but according to ICAO pilots should us one of the four categories and NOT " I am declaring an emergency" as non standard terminology can lead to errors , omissions or delays.
Retired Air Traffic Controller and Pilot
PS If interested a copy of ACP135(F) COMMUNICATIONS INSTRUCTIONS DISTRESS AND RESCUE PROCEDURES can be found at http://1drv.ms/MvlaTv