Flux.

Explanation

### Step-by-Step Text Solution for the Definition of Flux

1. **Understanding the Concept of Flux**:
- Flux is a term used in metallurgy and chemistry. It refers to a substance that is added to a mixture during a chemical process, particularly during smelting.

2. **Purpose of Flux**:
- The primary purpose of adding a flux is to facilitate the removal of impurities or gangue from the concentrated ore. Gangue refers to the unwanted materials that are mixed with the desired metal ores.

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