Definition of Jinn
The word of Jinn in Arabic refers to something that is concealed and hidden.
Al-Jawhari (was the aouthor of Al-Sihaah) Said: Al-Jaan is the father of the jinn, the plural of which is Jeenaan. It says in Al-Qamoos Al-Muheet: Jannahu Al-Layl means the night covered him.
Everything that is concealed from you is Junna ‘Anka (Concealed from you). Jinn Al-Layl means the darkness of the night. Al-Mijannah refers to the land where there are many Jinn. Al-Jaan is the plural of Al-Jinn (Al-Qaamoos Al-Muheet by Al-Fayroozabadi, pg.210.
It says in Lisaanul-‘Araf: Janana means to cover or conceal. Everything that is concealed from you is Junna’Anka (concealed from you). Jannahu Al-Layl means the night covered him… The Jinn are so called because they are concealed and hidden from view. Hence the foetus is called Al-Janeen because he is concealed in his mother’s womb (Lisaan Al-‘Arab by Ibn Al-Mudhir, Pg. 13, 92, 93.
Abu ‘Umar ibn ‘Abd Al-Barr said: According to the scholars of Arabic language, there are different categories of Jinn:
When they mention the Jinn in general terms, the use the word Jinni.
If the Jinn is one of those who live in houses with people, they call him ‘Aamir (dweller), the plural of which is ‘Ummaar.
If the Jinn is one of those who interact with children, they call them Arwaah (spirits).
If the Jinn is wicked then he is a Shaytaan (devil).
If the Jinn is worse than that, they call him Maarid (demon).
If the Jinn is worse than that and is strong, they call him ‘Ifreet, the plural of whis ‘Afaareet. (book: Aaakaam Al-Marjaan by Shinbli, pg 21.