Shidoin (指導員:しどういん) is a Japanese title, often used in budo. The word means instructor and is usually used to designate an official intermediate level instructor within an organization. A senior instructor would have the title Shihan.
Various budo arts and organizations have different requirements for the usage of the title, but in general it corresponds to 4th or 5th Dan. Some organizations also have the more junior title Fuku Shidoin which corresponds to assistant instructor. These titles are often separate from the dan rankings and are much more specific than the more general sensei.
The titles Fuku Shidoin, Shidoin and Shihan roughly correspond to the titles Renshi, Kyoshi and Hanshi, respectively, used by other martial arts organizations.
The Aikikai adopted this instructor designation system in the 1970s, around the time of the creation of the International Aikido Federation.