Further training of a pre-trained model on a narrower dataset to specialize or steer its behavior.
Broad technical consensus on what this term means. Disagreements that invoke it are about the world, not the word.
Fine-tuning takes a finished base model and continues training briefly on targeted data, examples of following instructions, a company's domain documents, a desired style. It is cheap relative to pre-training because the heavy general learning is already done.
It is the step that turns a raw next-token predictor into a usable product, and the standard lever for specialization. When a vendor offers "a model tuned on your data," fine-tuning is usually what is meant.