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PhD stands for Doctor of Philosophy. It's a doctoral degree awarded after years of rigorous academic study and research culminating in a significant original contribution to knowledge in a specific field. The "philosophy" in the name is a historical artifact; it originally referred to the pursuit of knowledge in general, and now encompasses a vast array of disciplines, from the humanities and social sciences to the natural sciences and engineering.
* MD stands for Doctor of Medicine. It's a professional degree awarded to physicians (doctors) after completing medical school and residency training. It signifies completion of the required education and clinical training necessary to practice medicine.