One at a time or all at once?

1. Which is correct one at a time or one at the time?
2. Is all at once correct?
3. What is meant by all at once?
4. Is it correct to say one and all?

Which is correct one at a time or one at the time?

"At the time" means in that exact moment. "At a time" means one by one.

Is all at once correct?

If a number of different things happen at once or all at once, they all happen at the same time. You can't be doing two things at once.

What is meant by all at once?

If something happens all at once, it happens suddenly, often when you are not expecting it to happen. I feel terribly sleepy all at once. All at once there was someone knocking on the door. Synonyms: suddenly, unexpectedly, abruptly, all of a sudden More Synonyms of all at once.

Is it correct to say one and all?

One and all means everyone present or everyone in a particular group.