To maintain understanding, usage, and interrelations of English vocabularies by Iraqi second language learners (L2) is a challenging mission. In the current study, the cognitive linguistic theory of domains by Langacker (1987) is adopted to provide new horizons in learning vocabulary and qualify Iraqi students with a deep knowledge analysis of the meanings of lexical concepts. This paper aims to test the validity of expanding the English language vocabulary for second language learners from Iraq through domains theory. It also attempts to find how the domains theory supports L2 learners in identifying meanings related to lexical concepts. Accordingly, an experimental study is conducted on fifty-eight university students of the second year level from the University of Baghdad, Iraq. The pre and post-tests are analyzed by using the Editor for the Statistical Package for Social Sciences (SPSS). The results show the following: First, a progression of more than (0.05=) is discovered in terms of students' understanding of the interrelationships between lexical concepts. Second, a new vision is dealt with to connect lexical concepts with their meanings according to the focus of the speakers using Langacker's theory. Third, domains theory (profile/ base organization, active zone, and the perceptual basis for knowledge representation) has proven effective in expanding Iraqi students' treatment and perception of semantic domains of English lexical concepts precisely.