Define the term critical frequency in relation to sky wave propagation of electromagnetic waves.

Explanation

### Step-by-Step Solution:

1. **Understanding the Context**:
- The term "critical frequency" is related to the propagation of electromagnetic waves, particularly in the context of sky wave propagation.

2. **Definition of Critical Frequency**:
- Critical frequency refers to the maximum frequency of radio waves that can be transmitted towards the ionosphere and still be reflected back to Earth.

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