(Sanskrit) A period of cosmic activity. Manvantara comes from two words:
Manu implies “a host of beings,” but we will say “minds” because from manu also we find the word “manas,” which means mind. But we have to understand that this type of mind is not like the mind that we use here in the three dimensional world. Manu is a type of intelligence that we call mind. Manu is precisely a group of minds or beings, a group of entities that enter into activity within the Ray of Okidanokh.
Antara means “within” or “in between.”
So the complete meanings is “the Manus, minds, or entities within” the Ray of Creation, within the Ray of Okidanokh. In synthesis, we will say: Manvantara refers to the great manifestation of all of those entities that emerged from the Ain Soph. Usually, manvantara is translated as "Cosmic Day." Notwithstanding, a Cosmic Day needs an intelligence in order to enter into activity.
Every manvantara is preceded and followed by a pralaya, a period of repose.
See also: mahamanvantara.