Upanishads

The term Upanishad is derived from upa (near), ni (down) and s(h)ad (to sit). In ancient Indian teaching system groups of pupils sit near the teacher to learn from him the secret doctrine. Upanishad means brahma-knowledge by which ignorance is loosened or destroyed.
 
It is here that we find all the fundamental teachings  that are central to Hinduism — the concepts of

'karma' (action), 'samsara' (reincarnation), 'moksha' (nirvana), the 'atman' (soul), and the “Brahman”  (Absolute Almighty). They also set forth the prime Vedic doctrines of self-realization,  yoga and meditation.