The word you are looking for is **jargon**.

> "I'm looking for a big word that can explain the specific knowledge
> that just certain amount of peolpe have. For example, When doctors
> talk they use an specific vocabulary for their area that doctors
> dominate."
> 
> "When doctors talk they use their **jargon** which is only understood
> by them."

So, doctors, for example, share a specialized vocabulary -- a **jargon** -- that it is easy for other doctors -- but difficult for non-medical professionals -- to understand.

From *The Free Dictionary*:

> **jargon**: the specialized language of a trade, profession, or similar group, especially when viewed as difficult to understand by
> outsiders

In general, different types of specialist have their own **jargon**, whether they be medical doctors, physicists, mathematicians, lawyers, etymologists, or grammatologists.

Even within areas such as medicine, physics, etc, practitioners specialize in sub-areas. In physics, for example, a physicist might specialize in condensed matter physics; solid state physics; quantum physics; statistical physics; or atomic, molecular, and optical physics, to mention only several of many possible sub-areas; each of these sub-areas has its own **jargon**. One could easily construct similar lists for medical doctors, mathematicians, lawyers, etymologists, and grammatologists.