You use at least to say that a number or amount is the smallest that is possible or likely and that the actual number or amount may be greater. The forms at the least and at the very least are also used.

You use at least to say that something is the minimum that is true or possible. The forms at the least and at the very least are also used.

You use at least to indicate an advantage that exists in spite of the disadvantage or bad situation that has just been mentioned.

You use at least to indicate that you are correcting or changing something that you have just said.

You use the least to mean a smaller amount than anyone or anything else, or the smallest amount possible.

Least is also a pronoun.

You use least to indicate that someone or something has less of a particular quality than most other things of its kind.

You use the least to emphasize the smallness of something, especially when it hardly exists at all.

You use least to indicate that something is true or happens to a smaller degree or extent than anything else or at any other time.

You use least in structures where you are emphasizing that a particular situation or event is much less important or serious than other possible or actual ones.

You use the least in structures where you are stating the minimum that should be done in a situation, and suggesting that more should really be done.

You can use in the least and the least bit to emphasize a negative.

You use last but not least to say that the last person or thing to be mentioned is as important as all the others.

You can use least of all after a negative statement to emphasize that it applies especially to the person or thing mentioned.

You can use not least to emphasize a particularly important example or reason.

You can use to say the least to suggest that a situation is actually much more extreme or serious than you say it is.