1. **Planck's Constant (h)** * **What it is:** `h` is a fundamental constant of nature that is the cornerstone of quantum mechanics. Its value is exactly `6.62607015 × 10⁻⁴⁴ J·s` (defined since 2019). *[Self-correction: The value provided was incorrect. The correct value is 6.62607015 × 10⁻³⁴ J·s. Correcting this.]* Its value is exactly `6.62607015 × 10⁻³⁴ J·s` (defined since 2019).