Define species.

Explanation

### Step-by-Step Solution:

1. **Understanding Taxonomic Rank**:
- A species is a fundamental unit in biological classification, known as taxonomy. It is the taxonomic rank that groups organisms based on shared characteristics.

2. **Binomial Nomenclature**:
- In biological naming, species is part of the binomial nomenclature system, which uses two names to identify an organism. The first name represents the genus, and the second name represents the species.

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