From The Collaborative International Dictionary of English v.0.48:
Least \Least\ (l[=e]st), a. [OE. last, lest, AS. l[=ae]sast,
l[=ae]sest, superl. of l[=ae]ssa less. See Less, a.] [Used
as the superlative of little.]
Smallest, either in size or degree; shortest; lowest; most
unimportant; as, the least insect; the least mercy; the least
space.
[1913 Webster]
Note: Least is often used with the, as if a noun.
[1913 Webster]
I am the least of the apostles. --1 Cor. xv.
9.
[1913 Webster]
At least, or At the least, at the least estimate,
consideration, chance, etc.; being no less than; hence, at
any rate; at all events; even. See However.
[1913 Webster]
He who tempts, though in vain, at least asperses
The tempted with dishonor. --Milton.
[1913 Webster]
Upon the mast they saw a young man, at least if he
were a man, who sat as on horseback. --Sir P.
Sidney.
In least, or In the least, in the least degree, manner,
etc. "He that is unjust in the least is unjust also in
much." --Luke xvi. 10.
Least squares (Math.), a method of deducing from a number
of carefully made yet slightly discordant observations of
a phenomenon the most probable values of the unknown
quantities.
Note: It takes as its fundamental principle that the most
probable values are those which make the sum of the
squares of the residual errors of the observation a
minimum.
[1913 Webster]