I'm looking for a word that can explain the specific knowledge that just certain number of people have. For example, when doctors talk, they use specific vocabulary for their field.
When doctors talk they use their [specific knowledge] which is only understood by them.
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.
There is the adjective esoteric which means, something likely known by only a few people, or a specific group of people. However, as Dan Bron commented, an esoteric vocabulary would be usually referred to as jargon.
terminology (M-W):
the technical or special terms used in a business, art, science, or special subject
Example: the terminology favored by sportscasters
lingo (M-W):
the special vocabulary of a particular field of interest
Example: The book has a lot of computer lingo that I don't understand.
parlance (M-W):
language used by a particular group of people
If a phrase will work instead of only one "big word", then perhaps domain expertise or subject matter expertise will work in your case. They're phrases in common use and can apply to different domains/disciplines broadly, not just medicine. See also
Also, these terms describe the specific knowledge and not the people themselves (e.g. subject matter experts). And while jargon refers to the terms these subject matter experts often use, the phrase mentioned in this post refers to the knowledge that those jargon terms are often used to describe.
lexicon OED
fig. the vocabulary proper to some department of knowledge or sphere of activity
As in:
1973 Times 31 July He [Mr. Ehrlichman] said the term ‘deep six’—meaning throw in the river—had not been ‘part of my lexicon’.
and
When doctors talk they use their medical lexicon, which is only understood by them.
Esoteric - Intended for or likely to be understood by only a small number of people with a specialised knowledge or interest. The opposite of this word is exoteric, which means intended for or likely to be understood by the general public.
Credentials: Musa Phungula studied Master of Commerce in Business Management Strategy-as-Practice
Expertise:
special skill or knowledge : the skill or knowledge an expert has - merriam-webster.com
Pansophism: It also has to do more specifically with pedagogic ideas of universal wisdom (pansophia), as it occurred in the educational system of universal knowledge.
Oligosophism: Has to do more specifically with pedagogical ideas, as it happens in the educational system of knowledge for few people (oligosophia).
