least

• something which is smallest in size or importance, something which is least
smallest in size; smallest in amount; lowest in importance
to the smallest extent possible; to the smallest amount or degree
you use least when comparing two or more things, to say that the amount of one of the things is the smallest of the two or of the group.
you also use least to say that something is true to as small a degree or extent as is possible.
you also use least to say that something has less of a particular quality than most other things of its kind.
you can also use least to emphasize that a particular situation or thing is much less important or serious than other ones.
you use at least to say that the number or amount mentioned is the smallest that is likely, and that the actual number or amount may be greater.
you also use at least to say that something is the minimum which should be done in the circumstances, although in fact you think that more than this ought to be done.
at least can also be used to indicate an advantage that exists in spite of the disadvantage or bad situation that has just been mentioned.
when you want to correct something that you have just said, you can also use at least.
you can use in the least and the least bit to emphasize a negative.
you can use to say the least to suggest that a situation is actually much more extreme or serious than you say it is.
you can use not least when giving an important example or reason.
you can use least of all after a negative statement to emphasize that it applies especially to a particular person or thing.