Define unit cell.

Explanation

**Step-by-Step Solution:**

1. **Definition of Unit Cell**: A unit cell is defined as the smallest repeating unit of a crystal lattice. It serves as the fundamental building block of the crystal structure.

2. **Crystal Lattice**: The arrangement of atoms, molecules, or ions in three-dimensional space within a crystal is referred to as a crystal lattice. The entire crystal is composed of numerous unit cells arranged in a specific pattern.

3. **Types of Unit Cells**: There are different types of unit cells based on their arrangement:
- **Primitive Unit Cell**: Contains lattice points only at the corners.
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