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Tapas means literally heat, afterwards any kind of energism, askesis, austerity of conscious force acting upon itself or its object. <ref>https://incarnateword.in/sabcl/18/the-origin-of-the-ignorance#p17</ref>

When the will and energy are concentrated and used to control the mind, vital and physical and change them or bring down the higher consciousness or for any other yogic purpose or high purpose, that is called tapasyā. <ref>http://incarnateword.in/cwsa/29/surrender#p71</ref>