What is the occurrence of different forms of the same compound called?

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the occurrence of different forms of the same compound. This indicates that we are looking for a term that describes this phenomenon.

2. **Identifying the Term**: The term that describes the occurrence of different forms of the same compound is "polymorphism."

3. **Defining Polymorphism**: Polymorphism refers to the ability of a compound to exist in more than one crystalline form. In other words, while the chemical composition remains the same, the arrangement of the atoms or molecules in the solid state can vary.

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