The word jinn in Arabic refers to something that is concealed and hidden.
Al-Jawhari said: Al-Jaan is the father of the jinn, the plural of which is Jeenaan. It says in Al-Qaamoos 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.
It says in Lisaanul-'Arab: 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.
Abu 'Umar ibn 'Abd Al-Barr said: According to the scholars of Arabic language, there are different categories of jinn:
1 - When they mention the jinn in general terms, they use the word Jinni.
2 - If the jinn is one of those who live in houses with people, they call him 'Aamir (dweller), the plural of which is 'Ummaar (spirits).
3 - If the jinn is one of those who interact with children, they call them Arwaah (spirits).
4 - If the jinn is wicked then he is a Shaytaan (devil)
5 - If he is worse than that, they call him Maarid (demon)
6 - If he is worse than that and is strong, they call him 'Ifreet, the plural of which is 'Afaareet