Why is prime number called ”prime number” ? Any reason ?

Some 2,000 years ago Euclid defined prime numbers in his definitive Geometry textbook The Elements (book 7, definition 11), as follows:


圖片參考：http://primes.utm.edu/notes/faq/gifs/EuclidsDef.gif


which is:

Prtos arithmos estin ho monadi moni metroumenos.

The Elements was the standard text for geometry in the West until the twentieth century; so it was the book that fixed our choice of terminology. Roughly transliterated, Euclid's definition is:

Prime numbers are that unit alone measured.

Or translated:

A prime number is that which is measured by an unit alone.

Rather than say 'measured,' we now say 'divisible,' but the idea is simple. Prime numbers are not multiples of other numbers (the unit one was not view as a number).
So why the word 'prtos,' and why should it be 'prime' in English?
The Greek philosophers used the word 'prtos' is the sense of first in order of existence (see Henry George Liddell, Robert Scott, A Greek-English Lexicon, proteros and prtos: B.I.3.c). This is one of the standard meanings of our 'prime' or 'primary.' In fact the English word 'prime' is from the Latin word for first: 'primus.' In a multiplicative sense prime numbers are thus the first numbers, the numbers from which the other numbers all arise (through multiplication). All other numbers (positive integers) are measured by primes, but primes alone are measured only by units. This makes primes first.
When did 'prtos' become 'prime'? In 1570, Sir Henry Billingsley first translated Euclid's Elements to English; forever establishing 'prime' are the correct English term for prime numbers.
In summary: we use the English word prime because the ancient Greeks saw them as multiplicatively first, so Billingsley translated Euclid's 'prtos' as 'prime'.

The property of being a prime is called primality, and the word prime is also used as an adjective. Since two is the only even prime number, the term odd prime refers to any prime number greater than two.