In mathematics, the phrase "almost all" has a number of specialised uses.

"Almost all" is sometimes used synonymously with "all but finitely many" (formally, a cofinite set) or "all but a countable set" (formally, a cocountable set); see almost.

A simple example is that almost all prime numbers are odd. (Two is a prime number.)

When speaking about the reals, sometimes it means "all reals but a set of Lebesgue measure zero" (formally, almost everywhere). In this sense almost all reals are not a member of the Cantor set even though the Cantor set is uncountable.

In number theory, if P(n) is a property of positive integers, and if p(N) denotes the number of positive integers n less than N for which P(n) holds, and if

p(N)/N → 1 as N → ∞

(see limit), then we say that "P(n) holds for almost all positive integers n" (formally, asymptotically almost surely) and write

\( (\forall^\infty n) P(n). \)

For example, the prime number theorem states that the number of prime numbers less than or equal to N is asymptotically equal to N/ln N. Therefore the proportion of prime integers is roughly 1/ln N, which tends to 0. Thus, almost all positive integers are composite (not prime), however there are still an infinite number of primes.

Occasionally, "almost all" is used in the sense of "almost everywhere" in measure theory, or in the closely related sense of "almost surely" in probability theory.
See also

Sufficiently large

References

Weisstein, Eric W., "Almost All" from MathWorld.