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
"When doctors talk they use their jargon which is only understood
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
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.