The term 'Thunder Sound' is a combination of three words, each of which describes a different sound associated with thunder. The first word is 'PEAL', which is a loud, ringing sound that is often heard during a thunderstorm. The second word is 'CLAP', which is a sharp, sudden sound that is usually heard immediately after a lightning strike. The third word is 'RUMBLE', which is a low, rolling sound that can be heard for a few seconds after a lightning strike. Together, these three words describe the various sounds associated with thunderstorms.