Koryu roughly translates to 'old flow' and is the Japanese term used to describe old styles and traditions, predominantly within the field of martial arts. The best conceptual translation into English is 'classical'.

A common dividing line between koryu styles based around bushi and samurai, and the the newer gendai budo styles of aikido, kendo, and judo, is the year 1868, when the Meiji restoration effectively ended the existence of the samurai. Occasionally koryu martial arts are classified as those used on the field of battle, which gives us a cut-off date in the late 1600's.

The major differences between the koryu and more modern styles is the structure of the ryu (school/style), in a koryu style there are usually only four ranks of practitioner; student, instructor, teacher and master rather than the usual eight 'belts' below shodan, and up to 9-10 above. The method of a ryu's succession was also very formalised. Each soke, or ryu 'headman' had his own style, and transmitted the secrets of his style to a few chosen disciples, one of whom would take over from him when it was deemed he had learnt all principles and techniques of the ryu. The designated heir to the ryu was often presented with a ceremonial scroll, or densho confirming his status.

Most of the modern martial arts are rooted in koryu, with Aikido stemming from Daito-ryu Aikijujitsu, and Judo coming from Jujutsu.