In Genesis, the Ark’s described opening in its side was a door.
Genesis 6:16 instructs Noah to make a door in the side of the Ark. The same verse mentions a lower, second, and third deck, and a window or opening near the top, depending on translation. The door is therefore the principal side entrance named in the construction instructions.
Genesis later says that Noah, his family, and the animals entered the Ark, and that the Lord shut Noah in. This wording gives the door a narrative role as well as a structural one. The text does not provide a measured width, hinge design, or detailed engineering plan for it.
Readers sometimes answer “window” because Genesis also mentions an opening near the top. That is a different feature. The question asks specifically about the opening placed in the Ark’s side, which Genesis calls a door.