To reenter means to enter something again after having left it. When astronauts return from space, their spacecraft must reenter Earth's atmosphere, passing back through the layers of air surrounding our planet. When you leave a room to grab something and come back, you reenter the room.

The word often appears in situations involving physical spaces: a student might reenter the classroom after visiting the nurse, or a player who was substituted out might reenter the game in the final quarter. But it also applies to activities and situations. Someone who stops working to raise children might reenter the workforce years later. A comet that swings around the sun and comes back toward Earth has reentered our part of the solar system.

The prefix re- means “again,” so reenter literally means “enter again.” Sometimes the word describes routine comings and goings, like reentering your house after getting the mail. Other times it marks something more significant, like reentering society after a long absence. A spacecraft's reentry can be a tense moment because the intense heat from friction with the atmosphere tests its protective shields.