Assumptions have been established that many machine learning algorithms expect the training data and the testing data to share the same feature space or distribution. Thus, transfer learning (TL) rises due to the tolerance of the different feature spaces and the distribution of data. It is an optimization to improve performance from task to task. This paper includes the basic knowledge of transfer learning and summarizes some relevant experimental results of popular applications using transfer learning in the natural language processing (NLP) field. The mathematical definition of TL is briefly mentioned. After that, basic knowledge including the different categories of TL, and the comparison between TL and traditional machine learning models is introduced. Then, some applications which mainly focus on question answering, cyberbullying detection, and sentiment analysis will be presented. Other applications will also be briefly introduced such as Named Entity Recognition (NER), Intent Classification, and Cross-Lingual Learning, etc. For each application, this study provides reference on transfer learning models for related researches.
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