"Yellow belt" is a ranking in a wide variety of martial arts systems. Usually it is the second lowest rank, just above white belt.
In aikido, yellow belt is typically the second lowest rank after white belt, although belt colors are not standardized in aikido, so the color ranks may differ between different dojo.
In Brazilian jiu-jitsu, yellow belt is a youth ranking - the third lowest, above white belt and gray belt and below orange belt. Yellow belt is further subdivided into white stripe ("beginner"), no stripe ("intermediate"), and black stripe ("advanced"). A child must be at least 7 years old to hold a yellow belt, and the belt is never be worn above the age of 15, at which time the youth graduates into the adult ranking system.
In karate, yellow belt is typically the second lowest rank after white belt, although belt colors are not standardized in karate, so colors may vary by school or dojo.
In krav maga, yellow belt is the second lowest rank, above white belt and below orange belt. Yellow belt is further sub-divided into the ranks of yellow belt-P1 ("practitioner 1") and yellow belt-P2 ("practitioner 2").
In kung fu, yellow belt is typically the second lowest rank, above white belt and below green belt. An advanced yellow belt practitioner will wear a yellow belt with a black stripe, before finally advancing on to green belt.
In judo, yellow belt is typically the second lowest rank after white belt, although belt colors are not standardized in judo, so the color ranks vary between different nations.