A practitioner of judo is known as a judoka or "judo practitioner", though traditionally only those of 4th dan or higher were called "judoka". The suffix -ka, when added to a noun, means a person with expertise or special knowledge on that subject. Other practitioners below the rank of 4th dan used to be called kenkyu-sei or "trainees".
OmegaWiki Dictionary
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judo A Japanese martial art and sport adapted from jujitsu.
