Define a taxon.

Explanation

**Step-by-Step Solution:**

1. **Definition of Taxon**: A taxon (plural: taxa) is defined as a unit of classification in biological taxonomy. It represents a group of one or more populations of organisms that are classified together based on shared characteristics.

2. **Classification Basis**: The classification of organisms into taxa is based on similarities and dissimilarities among them. This helps in organizing the vast diversity of life into manageable categories.

3. **Hierarchy of Taxa**: Taxa are arranged in a hierarchical order, which includes various ranks. The major ranks in this hierarchy are:
- Kingdom
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