In recent years, the emerging trend of mobile learning (hereinafter "m-learning") has obtained increasing attention from educators and higher popularity among students worldwide. Partly due to its short history, there seems to be a lack of comprehensive and in-depth understanding on the distinctiveness of m-learning. It is maintained in this paper that m-learning should be described and researched in three intertwined aspects: the mobility of devices, learners and materials. The multi-faceted mobility may help achieve highly portable and genuinely personal learning, which distinguishes m-learning from other forms of technology-enhanced education. This paper also reports an empirical study on m-learning for academic English. With a total of 60 participants (and six in the pilot study) over three progressive phases, the data obtained from mixed methods of automatic logging, questionnaire and interview were used to assess multidimensional learner engagement in the self-directed process of m-learning. The triangulated data can reflect and highlight some known features of m-learning, and they may also imply learners' expectations towards m-learning. Based on the literature review and data analysis, this paper aims to contribute to the (re)conceptualisation of "m-learning" and also provide some technical and pedagogical implications on future and further development of m-learning.