The basic unit of biological classification is ...................

Explanation

**Step-by-Step Solution:**

1. **Understanding Biological Classification**: Biological classification is a systematic method used to categorize living organisms into groups based on their similarities and differences. This helps in organizing the vast diversity of life.

2. **Identifying the Basic Unit**: In biological classification, different levels or categories are used, which are often referred to as taxonomic ranks. These ranks include domain, kingdom, phylum, class, order, family, genus, and species.

3. **Defining 'Species'**: The term 'species' refers to the most specific level of classification. It is defined as a group of individuals that share common characteristics and can interbreed to produce fertile offspring. Members of a species are usually similar in appearance and behavior.

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