Asymmetric carbon atom.

Explanation

### Step-by-Step Solution:

1. **Definition of Asymmetric Carbon Atom**:
An asymmetric carbon atom, also known as a chiral carbon atom, is defined as a carbon atom that is bonded to four different groups or atoms. This unique arrangement allows for the existence of two non-superimposable mirror images, known as enantiomers.

2. **Characteristics of Asymmetric Carbon**:
- The carbon atom must be tetravalent, meaning it forms four bonds.
- Each of the four groups attached to the carbon must be different from one another. This difference is crucial for chirality.
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