Detailed Explanation:

Galileo Galilei was the first to define speed quantitatively as distance covered per unit of time.

• Historical Context: Galileo introduced the concept during his studies on motion and kinematics in the early 17th century.
• Significance: He established speed as a measurable scalar quantity, laying the foundation for classical mechanics.
• Therefore, Galileo is credited with the first definition of speed, making Option 4 the correct answer.