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The term 'Outsiders' has been used with two separate meanings within the Fatal Frame Series. While separate in definition, both meanings describe a visitor or traveler who is visiting a certain location.
In the Minakami Village setting of the second game, the term outsider is used as one of many names for visitors to the village. These visitors may be ultimately used as Kusabi sacrifices to calm the Hellish Abyss. Such as in the case of Seijiro Makabe.
While not all outsiders to the village were used as Kusabi, only outsiders would be used as Kusabi sacrifices (save for the first Kusabi who was a ceremony master.) This occurred in between Crimson Sacrifice Ceremonies when twins weren't available to be offered.
Due to the status of being an outsider, it was forbidden to speak to outsiders about the village. (This may have included not only the rituals but recent village events, as in the case of Itsuki's imprisonment.) 
- Ryozo's father
In the third game, "outsiders" is used as a term given to the men that are invited every few years to the Kuze Shrine to reproduce with a female member of the Kuze Family. This is done in hopes of producing a female heir to continue the Kuze family bloodline.
An outsider is invited as a guest to stay at the Kuze Shrine until the snows of winter melt. During their stay, an outsider is only allowed into certain rooms, probably those that a Tattooed Priestess does not enter. If a priestess were to see these male guests, their heart would stir and their ability to be a worthy priestess would be compromised. During the day the outsider is not allowed to leave their guest room. At night they are ushered to the chambers of the female member of the Kuze Shrine, where they are encouraged to mate with them.
At the end of winter, the outsider is either cast out of the manor never to return, or, more likely, they are killed and disposed of. Even if the Outsider is allowed to leave without being killed, they will certainly be killed if they try to return to the shrine.
- Kyouka's father
- In the Japanese version of Fatal Frame II, Outsiders are known as "marebito" (客人) - literally meaning "visitor". It is also a term used in Japanese folklore.
- The Japanese term used in Fatal Frame III, "niinae", can mean "new seedling/sapling", which may refer to how the men were used to produce children.