Grandmaster (or Grand Master) and Master are titles used to describe or address some senior or experienced martial artists. Typically these titles are honorific in nature, meaning that they do not confer rank, but rather distinguish the individual as very highly revered in their school, system, or style.
Asian martial arts traditionally use terms that are usually translated as "teacher" and the use of "" was a Western invention derived from 1950s United States war veterans returning home with stories of the incredible martial feats of certain individuals and groups. Subsequently, they found their way into martial arts culture as marketing tactics to the extent that the titles are aligned to the 'elderly martial arts master' stock character. In Asian countries, such titles are more commonly reserved for religious leaders and saints.
The use of "master," "grandmaster," etc. is decided within an individual art or organization. The use may be self assigned; for example having promoted a student to 'teacher' level, or may be assigned by a governing body in arts with a more formalised structure, and some do not use it at all, for historic reasons or to avoid the 'elderly master' stereotype. The modern use of Dan rankings and Black belt and Red belt in martial arts both derive from Judo where they were adopted by its founder Kanō Jigorō.
There are many terms similar or equivalent to 'grandmaster' used by various martial arts traditions. Some of these terms derive from older systems, while others are relatively modern.
Japanese martial arts commonly use Sensei (先生) meaning "teacher" or literally translated, "born first" or "one who has gone before". A Sensei is a person who has knowledge and is willing to teach that knowledge to another. A Sensei assists students in ken shiki "the pursuit of knowledge". Some organizations, such as the Bujinkan, Kodokan (Judo), and Shodokan Aikido, use the term shihan for high-ranking or highly distinguished instructors. Sōke (宗家?), meaning "the head family [house]," is sometimes used to refer to "founder of a style" because many modern sōke are the first generation headmasters of their art, but most correctly refers to the current head. A sōke is considered the ultimate authority within their art and has the authority to issue a menkyo kaiden certificate indicating that someone has mastered all aspects of the style.