Idioms, commonly considered to be a figure of informal, colorful, and entertaining language, are supposed not to lend themselves to formal discourses. However, idioms have recently been found to be employed in formal communications and academic discourse. Along the same lines, this corpus-based study, for the first time, strived to shed new light on English idioms by investigating them in a corpus of 5,675,554 tokens of 640 randomly selected applied linguistics research articles (RAs). To this end, an operational definition and framework for the definition and identification of academic idioms in written academic genres such as RAs were developed. Then, based on the framework, a list of the most frequent academic idioms in this genre was compiled. Moreover, any possible changes in the frequency of this group of idioms during 1980-2020 were investigated. The results of the study indicated that idioms are used in applied linguistics RAs and deserve to receive apt attention. It was also found that there is not any significant relationship between time and the frequency of idioms in RAs. The style of the writers seemed to play a crucial role in the frequency of idioms. By studying and providing the most frequent academic idioms in applied linguistics RAs, this study's findings might prove useful for the EFL writing instructors, applied linguistics graduate and post graduate students, and teaching materials developers.
