नियतं सङ्गरहितमरागद्वेषतः कृतम्। अफलप्रेप्सुना कर्म यत्तत्सात्त्विकमुच्यते ॥

niyataṁ saṅga-rahitam arāga-dveṣhataḥ kṛitam aphala-prepsunā karma yat tat sāttvikam uchyate

An action which is ordained, done by one undesirous of fruit, devoid of attachment, without love or hate, that is called pure.

In plain words: A pure action is one that is your proper duty. You do it without craving the reward, without clinging, and without liking or hating anyone.