GPT in ChatGPT stands for Generative Pretrained Transformer. The term "Generative" refers to the ability of the model to generate text, while "Pretrained" means that the model has been trained on a large corpus of text data prior to being fine-tuned for specific applications. The "Transformer" part of the name refers to the architecture of the model, which is based on the transformer architecture. The transformer architecture is a deep learning model that has been highly successful in a wide range of natural language processing tasks, making it well-suited for language generation tasks like those performed by ChatGPT.