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One

One (1, also a unit) is the first positive whole number, signifying the existence of a single unique object we're counting.

Some facts about this number include:

It is an odd number.
It is a positive number, whole number (integer), real number, rational number and complex number.
It is by convention NOT a prime number, though it is only divisible by 1 and itself.
It is a multiplicative identity, i.e. 1 x = x* for any number x. Also x / 1 = x, x^1 = x, 1^x = 1 for any number x.
In programming there is a very common type of bug called off by one in which a boundary is either incorrectly included or excluded.
1 is often a convenient upper bound of many intervals, e.g. when normalizing numbers or dealing with probabilities.
In computing the number and digit 1, as opposed to 0, usually means the true or on value.
In programming operations with 1 (similarly to 0) are very common and may sometimes be handled as special cases to help efficiency, for example adding or subtracting one is called incrementing and decrementing and many assembly languages have special instructions for this.