GPT stands for "Generative Pre-trained Transformer." It's a type of artificial intelligence model developed by OpenAI. The name breaks down as follows:

1.  Generative : The model can generate text and other forms of data. It's capable of producing coherent and contextually relevant text based on the input it receives.

2. Pre-trained : Before being fine-tuned for specific tasks, GPT models are trained on massive amounts of text data from the internet. This pre-training helps the model learn grammar, language structure, and even some level of common sense.

3. Transformer : The "Transformer" architecture is a key innovation in neural network design. It allows the model to process and generate text in parallel, making it highly efficient. It also employs self-attention mechanisms to understand the relationships between different words in a sentence, which is particularly effective for tasks involving context and coherence.

In the case of ChatGPT, it's a version of the GPT model that has been fine-tuned for generating human-like text in a conversational context. It can understand and respond to user inputs, making it suitable for tasks like chatbots, customer support, content generation, and more. GPT-3.5, the specific version mentioned earlier, is one of the iterations in the GPT series, and it builds upon the improvements made in the earlier versions to produce even more coherent and contextually relevant text.