The definition of 9 is a numerical value that comes after eight and before ten. It is the highest single-digit number in the decimal system, which has been used since ancient times. Nine is also considered to be an important symbol in many religions and cultures throughout history due to its divisibility by three, making it a powerful representation of completeness or wholeness.

In mathematics, nine can be seen as an odd number with factors 1, 3 and 9 itself; this makes it unique among single-digit numbers as it cannot form any integer other than itself when multiplied by another single digit number. In addition to this distinction from other numbers in its class (the natural numbers), nine also stands out for being the only perfect square between one and twenty four (4 x 4 = 16). This property allows us to use certain formulas involving square roots more easily when dealing with problems related specifically to 9 or multiples thereof such as 18 or 27 etc..

Nine has long held special significance across multiple disciplines including astrology where it represents wisdom; music where there are numerous references made towards ‘nine’ within song lyrics; literature too often makes mention of ‘nine’ either directly through character names/places etc., or indirectly through themes associated with completion/wholeness such as The Lord Of The Rings trilogy wherein Frodo must complete his quest on behalf of Middle Earth’s inhabitants at Mount Doom – known colloquially amongst fans simply as ‘Mount Nine’.

All these examples serve further testament not only towards how deeply embedded into our culture(s) this simple yet powerful concept truly lies but how much meaning we have ascribed onto what was originally just another arbitrary mathematical figure over time!