The present paper is an attempt to understand the essence of English for Specific Purposes (ESP) by studying various language specialists’ efforts to define it, tracing its evolution and presenting its distinctive characteristics. Being a branch of English Language Teaching (ELT), its focus is narrower than that of EGP (English for General Purposes) and in contrast to EGP, ESP is determined by specific learning needs of the language learner. Although, there have been contradictions as to what ESP is, Hutchinson and Waters improved absolute and variable characteristics helped significantly in resolving arguments about what is ESP. Thus, ESP refers to the teaching and learning of English where the aim of the learners is to use English in a particular academic, professional or occupational domain, focusing on learners’ special needs.