Aspect term extraction is a natural language processing task that automatically recognizes and extracts aspect term from free expression text. The study first goes over the basic task of aspect term extraction, the authoritative datasets of it and general evaluation specifications on it. Based on these, the study comprehensively reviews on the state-of-the-art techniques for the task, including traditional extraction techniques based on statistical strategies and feature engineering, and the neural extraction techniques using deep learning. In particular, the study takes the essence of expression language as the starting point, combines with the limitations of existing techniques and gives an elaboration of the technical difficulties and the future development prospects of aspect term extraction.