Tamas is, in the Samkhya philosophy and the Bhagavad Gita, the third of the three gunas, the strand of inertia, darkness, heaviness and dullness, the quality of resistance, obscurity, sleep and decay, that which conceals and weighs down. In the threefold analysis of nature it is the principle of stability at its densest and of the veiling of knowledge, associated with ignorance, lethargy, delusion and the downward pull toward the inert.
The Bhagavad Gita describes tamasic food, action and temperament as those marked by dullness, negligence and confusion, and it presents tamas as binding the soul through heedlessness and sloth. Yet like the others it has its necessary place, for it is the ground of rest, of matter's persistence and of the form that holds things together; the three gunas are always found in combination, and it is their shifting balance that constitutes all of manifest nature. The aim of the spiritual path is to rise through and finally beyond the three. The doctrine is ancient and central to Hindu thought.
Sources Classical Samkhya: An Interpretation of Its History and MeaningGerald James Larson with Srimad-Bhagavad-Gita, English translation and commentarySwami Swarupananda