Define the term "threshold frequency", in the context of photolelectric emission.

Explanation

**Step-by-Step Solution:**

1. **Understanding Photoelectric Emission**:
- Photoelectric emission refers to the phenomenon where electrons are emitted from a material (typically a metal) when it is exposed to light of a certain frequency.

2. **Role of Frequency**:
- Not all frequencies of light can cause photoelectric emission. There exists a specific frequency threshold below which no electrons are emitted, regardless of the intensity of the light.

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