ESP all essentially emphasize to language centred approaches and to give a definition of ESP we need to establish a context about how ESP at the present time relates to the rest of ELT.
In the time honoured manner of Linguistics, we shall represent the relationship in the form of a tree. They represent some of the common divisions that are made in ELT. The topmost branches of the tree show the level at which individual ESP courses occur. This level divided into two main types of ESP according to whether the learner requires english for academic study, they are EAP (English for Academic Purpose of for work/training) and EOP,EVP,VESL (English for Occupational Purpose/English for Vocational Purpose/Vocational English as a Second Language)
At the next level down, ESP courses distinguished by the general nature of the learner's specialism. Three categories are identified here, they are : EST (English for Science and Technology), EBE (English for Business and Economics) and ESS (English for the Social Sciences).
Next level, we can see that ESP is just one branch of EFL/ESL which are themselves the main branches of English language teaching in general. ELT is one variety of the many possible kinds of language teaching. As we know that, a tree cannot survive without roots. In this case, the roots which nourish the tree of ELT are communication and learning. The analogy of a tree above showing to us what ESP isn't. Here they are :
a. ESP is not a matter of teaching ''specialized varieties'' of English because the fact that language is used for a spesific purpose. There are some features which can identified as ''typical'' of a particular context of use, and which , so, the learners is more perhaps to meet in the target situation.
b. ESP is not just a matter of science words and grammar for scientist, so on. When we look at the tree, there's actually much hidden from view inside and beneath the tree although we know the leaves and the branches. They are supported by a complex underlying structure. The point is we need much more communication than just the surface features of what we read and hear and also we need to distinguish between performance and competence in relation to what people actually do with the language and the range of knowledge and abilities which can enables them to do it.
c. ESP is not different in kind from any other form of language teaching. It based on the principles of effective and efficient learning. Eventhough the content of the learning is different, the proccesses of learning should be any different for the ESP learner than for the general english learner.
ESP is different from the form of ELT because ESP must be seen as an approach not as a product. ESP is not a particular kind of language or methodology, nor does it consist of a particular type of teaching material. ESP is an approach to language learning, which is based on the learners' need. So, we conclude that ESP is an approach to language teaching in which all decisions as to content and method are based on the learners' reason for learning.
Conclusion : The main factors in the origin of ESP that the Linguistic factor dominate the ESP’s development that focus on the nature of specific varieties of language use. In tree of ELT, it shown that they are primarily concerned with communication and learning. ESP is not a language product but as an approach to language teaching which is directed by specific and apparent reasons for learning.