Abstract Despite going through some ups and downs, grammar teaching has always been one of the central issues in the history of second language teaching. In order to teach grammar, teachers frequently get involved in metalanguage, which has grammatical terminology as one of its major components. Since the nature and use of grammatical terminology in language teaching has remained a considerably under-researched area to the day, the present study, originally a doctoral dissertation, was an attempt to find a difficulty index for a more or less comprehensive list of English grammatical terms, collected from various sources of English grammar. For this purpose, frequency of terms in a researcher-built corpus of EFL/ESL pedagogic grammar textbooks and English students’ familiarity with the terms were used as the two main criteria for calculating the difficulty index. A corpus of 14 grammatical textbooks was created, and then each of the 459 terms in the list was searched for in the textbooks to calculate their frequencies as well as ranks in the corpus. Student familiarity with the terms in the list was also measured through a productive test of grammatical terminology administered to 72 BA students of English at Shahid Beheshti University in Iran. Based on the results, the traditional dichotomy of scientific versus pedagogic terminology was questioned, arguing for an additional category, non-pedagogic term. Accordingly, 173 (37.7%) of the terms in the list never appeared in the corpus and thus were labelled non-pedagogic. Terms with a large corpus/test rank were reanalyzed to find out about the reasons for the gap. Furthermore, the distribution of terms across the corpus textbooks revealed that as the level of the books rises, the number of terms also increases, indicating the direct relationship between second language proficiency and metalingual knowledge. Most importantly, more than 10 major and minor trends in the use of grammatical terminology in pedagogy were explored and suggested. Finally, as the output of the study, 6 equivalent objective tests of pedagogic grammatical terminology were developed for the first time in the literature.