What is meant by a substance ?

Explanation

### Step-by-Step Solution:

1. **Definition of Substance**: A substance is a form of matter that has a uniform and definite composition. This means that a substance is made up of only one type of particle.

2. **Composition**: Substances have a specific composition, which means that the elements or compounds that make up the substance are present in a fixed ratio. For example, water (H₂O) is a substance that is always made up of two hydrogen atoms and one oxygen atom.

3. **Properties**: Substances possess specific properties that distinguish them from other types of matter. These properties can include physical characteristics like color, melting point, boiling point, and density, as well as chemical properties.

...