Brahman is the ancient concept, found in the vedas.

Meaning of brahman –

The word ‘brahman‘ means, the highest or biggest.

According to the vedas and upanishads –

Brahman is the one who is big, great and noble

He is regarded as the Brahman, who has entered into all beings
– Maitreya Upanishad 6.38

According to the vedic philosophy – 

It is the highest principle, reality & the mystery amidst and beyond the universe, which is beyond everthing and cannot be exactly defined.