Albert Einstein formulated the theory of special relativity, which includes the postulate that the speed of light in a vacuum is constant and independent of the motion of the observer, in the year 1905. This theory is outlined in his paper titled "On the Electrodynamics of Moving Bodies," also known as the special theory of relativity paper.
In this groundbreaking paper, Einstein introduced the concept of the constancy of the speed of light and showed that it has profound implications for the nature of space and time. This work revolutionized our understanding of physics and laid the foundation for modern physics and our understanding of the universe.
It is important to note that while Einstein's work was revolutionary, the concept of the constancy of the speed of light had been hinted at and studied by other scientists before him, such as the experiments conducted by Michelson and Morley in 1887. However, Einstein's formulation and theoretical framework provided a new and comprehensive explanation of these phenomena.