How do you define the octet rule?

Get 8 outer electrons

Here is the definition that I use : The states that atoms tend to gain or lose electrons in their outer levels until they acquire eight electrons and are then unreactive.

Ultimately, the atoms become more stable if they can get a full energy level. Helium has a full energy level with 2 electrons so it doesn't follow the octet rule. Any atoms that try to get the of helium (lithium, beryllium and perhaps boron) will lose electrons to get to the electron configuration of helium and they also won't follow the octet rule.

Other will be most stable if they can either end up with a filled energy level or have 8 outer electrons. This explains why Group 17 elements try to gain one electron to go from 7 outer electrons to 8 outer electrons. Elements of Group 1 below lithium will lose one electron in order to empty their outer energy level so that they will then have 8 outer electrons and in the case of sodium, it will have a full second energy level.

So when an atom had the electron configuration of a noble gas, it doesn't easily gain or lose any more electrons. In most cases, it has satisfied the octet rule.