A large model trained on broad data that serves as a base for many different tasks. Claude, GPT, Gemini, and Llama are all foundation models. They're "foundational" because they can be adapted to almost anything — writing, coding, analysis, image understanding — without being specifically trained for each task.
Why it matters
Foundation models changed the economics of AI. Instead of training a separate model for every task, you train one massive model once and then fine-tune or prompt it for specific needs.