Glossary Part 1: Terms for the Supernatural & Occult
Note: You'll find that different sources don't always agree as to what constitutes a yōkai vs yuurei, a monster versus a demon, or even a dead vs living spirit. Japanese folklore is old and informed by religious practices, cultural traditions, and many pther factors. Even among characters in Modern Kaidan Romance, terms for supernatural entiries are used interchangabley.
あやかし Ayakashi (loosely, ghosts, but can refer to other kinds of supernatural entities)(sometimes it is used to refer to seafaring ghosts in particular)
化け物 Bakemono/お化け Obake (monsters, specifically shapeshifters, but it can refer to monsters or ghosts in genral)
生霊 Ikiryō (living spirit) - umbrella term for all living spirits, humans, animals, plants, kami, and youkai included, sometimes a human's living spirit can leave their body to haunt/curse others
怪異 Kaii (strangeness, mystery, oddity) - this is the term used as a catch-all for any kind of supernatural being or incident, usually referred to in English as "aberrations" or "oddities"
神 Kami (loosely, god or spirit) (Izanami, Benihime, and Doikawa-kamisama are all kami)(while the word is translated as "god," these entities are not all powerful; kami in the heavenly realm, like Amaterasu, are referred to as Heavenly Kami or the Heavenly Court)(The number "8 million" was used historically as a way to say that there were an uncountable number of kami)
汚れ Kegare (impurity - Shintoism often focuses on a dicotomy between "pure" and "impure;" kegare accumulate naturally over a person's lifetime and in nature, but must be occasionally cleansed to live a good life)
怨霊 Onryō (vengeful ghost attached to a human) - a specific type of ghost that latches on to a living human (Ibuki is an onryou)(examples in media include Sadako from Ring-u and Saeki Kayako from Ju-on).
式神 Shikigami (familiar spirit) - a spirit created out of inanimate materials to serve the person who created it (the familiars created by the Isshiki family and Takara Takuto are shikigami; Minami and Ryuuzaki a are also shikigami created by Ibuki)
死神 Shinigami (death god) - grim reaper-like figures that escort the souls of the dead to the underworld and act as messengers for Izanami-no-Mikoto (Inunaki, who Junna meets in the second chapter, is a shinigami)
死霊 Shiryō (dead spirit) - umbrella term for all dead spirits
付喪神 Tsukumogami - yōkai created from inanimate objects come to life, they are considered living spirits and sometimes kami
妖怪 Yōkai (apparitions) - another catch-all term for demons, they can be any kind of spirit, including ghosts; they can be compared to the Fair Folk of western fame
幽霊 Yuurei/Yuuki/Yuukai (ghost) - terms used to refer to ghosts in general, usually they are stuck haunting a certain location; Junna considers them easier to mediate with
Links include kanji information from Tangorin; terms are loosely defined in general with context from their use in-story
Glossary Part 2: Religion & Spiritual Practices
Shintō:
Jinja (Shrines):
Torii: the distinctive gates to shrines. often painted red
Kami: a term used to describe gods and/or spirits in Japan
Yomi: the underworld in Shinto cosmology, an unpleasant and dark place
Onmyōdō: techniques and practices that use yin and yang and the five Chinese elements for divination and exorcisms, draws from Daoism, Buddhism, Confucianism, Shintō, and Goryō (ancestor worship)
Onmyōji: a practitioner of onmyōdō, they used to hold positions in royal court as diviners