明神 (pronounced as 'myojin') is an honorable title used in Japanese Shinto for gods.
(When the word '明神' is used to describe Emperors, it is pronounced as 'Akitsumikami.'
Please consult the word '現人神' (Arahitogami).)
In Yoshida Shinto, 'myojin' is used as "shingo" (a title given to a deceased person who becomes a god). A famous example is 'Hokoku Dai-myojin' (Great Luminous Deity of Our Bountiful Country), which was given to (the deceased) Hideyoshi TOYOTOMI. The word 'myojin' implies that gods appear not as their tentative self but as their real self with a clear form.
On the contrary, the word '権現' (pronounced as 'gongen'), shingo used in Sanno Ichijitsu Shinto, a Buddhist school of Sanno Shinto, implies that gods appear as '権 (gon)', or tentative, self, or that Buddha appears as '権,' or a tentative, form of gods. A famous example is 'Tosho Dai-gongen,' which was given to (the deceased) Ieyasu TOKUGAWA.
During the period when Buddhism was disseminated among countries, some countries saw conflict between Buddhism and their local gods, but Japan incorporated Buddhism by concluding that gods would protect Buddhists (syncretism of Shinto and Buddhism). As a result, deities have been worshipped while being called '-myojin' in one time and '-gongen' in another time.
Upon the enactment of the Ordinance Distinguishing Shinto and Buddhism in the early Meiji period, people were prohibited from officially use the word 'myojin' together with other words such as 'gongen' and "Hachiman Daibosatsu" (Great Bodhisattava Hachiman).
Incidentally, the origin of the word 'myojin' is a Buddhist word.