Define thermoplastics.

Explanation

### Step-by-Step Solution:

1. **Definition of Thermoplastics**:
- Thermoplastics are a type of polymer that can be repeatedly softened by heating and hardened by cooling.

2. **Characteristics**:
- They can undergo multiple cycles of heating and cooling without undergoing any significant change in their chemical composition or mechanical strength. This means that their properties remain consistent even after being reshaped.

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