How did the concept of brahman emerge in the Upanishads?

In the Upanishads, the concept of brahman emerged as a shift from its earlier Vedic meaning—which signified the creative power of ritual deeds and sacred words—to a philosophical designation for the supreme, universal essence underlying the cosmos.1

The Upanishads sought to identify a single, unifying principle behind the apparent diversity of the universe. This led to the foundational equation of brahman with atman (the individual self). By identifying the Absolute Reality of the universe with the innermost core of the human person, the Upanishads spiritualized the concept of brahman and universalized the concept of atman.23

Through this identification, brahman came to be defined as the eternal, infinite, and irreducible spiritual core of all existence. While the Upanishads do not present a single, unified doctrine, they collectively describe brahman as the "one Being" that bursts forth into the manifested world, existing both as a transcendent reality and as an immanent presence within all beings. This realization of the connection between the individual and the cosmos remains the core teaching of many Vedanta traditions.432