French mathematician who created the first effective theory of the measure of sets of points and who shares credit with René-Louis Baire (Baire, René-Louis) and Henri Lebesgue (Lebesgue, Henri-Léon) of France for launching the modern theory of functions of a real variable.

The son of a Protestant pastor, Borel exhibited his mathematical talent from a young age. After placing first in the 1889 entrance exams for the École Normale Supérieure and the École Polytechnique, both in Paris, he decided that the former was the best avenue to an academic career. He graduated first in his class of 1893 and then taught at the University of Lille, where he wrote his thesis and 22 papers in the following three years before he joined the faculty of the École Normale Supérieure.

Borel discovered the elementary proof of Picard's theorem (see Charles-Émile Picard (Picard, Charles-Émile)). This sensational accomplishment set the stage for his formulation of a theory of entire functions and the distribution of their values, a topic that dominated the theory of complex functions for the next 30 years.

Although Borel was not the first to define a conventional sum of a v series (a series of numbers that does not approach a certain number; see infinite series), he was the first to conceive and develop a systematic theory of such series (1899). In 1909 he was appointed to the chair of theory of functions created for him at the Sorbonne. He completed a series of papers on game theory (1921–27) and became the first to define games of strategy.

Borel also served in the War Office during World War I, in the French Chamber of Deputies (1924–36), and as minister of the navy (1925–40). After his arrest and brief imprisonment under the Vichy regime during World War II, he returned to his native village and worked in the Resistance. For this work he was awarded the Resistance Medal (1945) to add to his Croix de Guerre (1918) and, later, the Grand Cross of the Legion of Honour (1950). He also was awarded the first gold medal of the National Centre of Scientific Research (1955).