A foundation model is a large-scale AI model trained on massive, diverse datasets that can be adapted to a wide range of tasks through prompting, fine-tuning, or tool integration. Models like GPT-4, Claude, Gemini, and Llama are foundation models. They encode broad general knowledge and language capabilities that can be specialized for specific use cases — chatbots, code generation, content writing, data analysis, and more. Foundation models are the core infrastructure of modern AI applications.