In the UK, the term the naughty step- the bottom step of the stairs- is often used. This term is much easier to use in conversation. You can say that a child is on the naughty step, off the naughty step, you can tell a child to go to the naughty step or to get off the naughty step.
This document uses the term "time in" for the state of not being in time out, but it appears in apostrophes which suggests that they do not consider it to be a real term.
This guide suggests that you can release a child by saying
You're being quiet, so you may come out of time out now
Many, however, feel that this casts the parent in the role of jailer. Other guides like this one therefore recommend going to the child at the end of the time out and reminding them why they were placed on time out, then telling them they need to say sorry for their behavior. When they do apologize, the parent is nice to the child and does not mention the time out again.