A number which was eaten by 7

Short for ultimate bias. Typically used by fangirls and fanboys to describe their favourite celebrity amongst the many that they are fans of.

A firearm chambered in 9mm. Properly used, it refers to the 9x19mm cartridge, aslo known as 9mm Parabelum or 9mm Luger. Though several other calibers are 9mm in diameter, they should not correctly be refered to as "nines".
9x17 AKA 9K AKA 9mm Kurtz AKA 9mm short is properly refered to in ghetto slang as a .380, referring to .380ACP the proper American nomenclature.
9x18 AKA 9mm Makarov is not commonly found in North America, and is therefore unavailable to most criminals.
9x21, is a more powerful variation of the 9x19 cartridge, and the two are not compatible in autopistols.
9x23, is a powerful cartridge, but is seldom found in cheap pistols, and therefore out of reach for most criminals.

When you're on IM or texting and a parent or other bothersome person is breathing down your neck, type "9" and the person who you are talking to will understand and switch to a different, more boring topic, like school or work.

9mm pistol carried by a fellow emc-er

The most basic inference of 9 is Death, or with the other angle Rebirth, like a phoenix.
The simplest usage of 9 is at The End of every digited set of natural numbers
Example: 1-9, 10-99, 100-999, 1000-9999, 10000-99999, etc, and thus the analogy with Death.
But as there is always a new bigger set after 9, it is also similar to Rebirth.

Different than a 9mm in a few ways.

-9s are fired sideways, rarely hitting anything. Used by street gangs.
-9mms are fired properly and hit where they are fired most of the time. Used by human beings.