Tapas means deep meditation, reasoned self discipline and effort to achieve self-realization, sometimes involving solitude, hermitism or asceticism; it is derived from the word root tap which depending on context means "heat" from fire or weather, or blaze, burn, shine, penance, pain, suffering, mortification.
In the Vedic literature of Hinduism, fusion words based on tapas are widely used to expound several spiritual concepts that develop through heat or inner energy, such as meditation, any process to reach special observations and insights, the spiritual ecstasy of a yogin or tāpasa (a Vriddhi derivative meaning "a practitioner of austerities, an ascetic"), even warmth of sexual intimacy. In certain contexts, the term is also used to mean penance, suffering, austerity, pious activity, as well as misery. The fusion word tapasvini, for example, means a female devotee or pious woman, "an ascetic, someone practicing austerities", or in some contexts it can mean poor, miserable woman.
In the yogic tradition, Tapas is the fire within and internal focus towards the goal of enlightenment, to foster self-control, one mindedness, simplicity, wisdom, integrity.
Example via www.ramdass.org: About Truth