A model invoking external functions (search, code execution, APIs) instead of answering from its weights alone.
Broad technical consensus on what this term means. Disagreements that invoke it are about the world, not the word.
In tool use, the model emits a structured call ("search for X", "run this code", "query this database"), the surrounding system executes it, and the result is placed back into the model's context to continue from. The model stops being a closed text predictor and starts acting on external systems.
Tool use is what extends models past their frozen knowledge and unreliable arithmetic, and it is the substrate agents are built on, each tool is a capability the model can decide to reach for.