In the Genesis flood narrative, the Ark's main opening was called a door.
Genesis 6:16 instructs Noah to make a door in the side of the Ark. Genesis 7 then says that Noah, his family, and the animals entered, and that the Lord shut the door behind them. The wording describes an opening in the side rather than a named roof hatch or a pair of gates.
The same verse also mentions a window or opening near the top of the Ark. That upper opening is associated with Noah's later action of sending out the raven and dove. Readers sometimes merge the two features because both are openings, but Genesis distinguishes the side door from the upper opening.
Translations differ in how they render some details of the Hebrew text, especially the upper opening. The door, however, is consistently the principal named entrance in common English translations. It is separate from the Ark's three internal decks.