In plain words
Steadfastness in yajña, tapas, and giving is also called ‘Sat’; and action performed for the sake of that is likewise called ‘Sat.’
Word for word
And steadfastness in yajña, in tapas and in dāna is also called 'Sat'; and action meant for that, too, is named just 'Sat'.
यज्ञे तपसि दाने च स्थितिः सदिति चोच्यतेकर्म चैव तदर्थीयं सदित्येवाभिधीयते
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
यज्ञे यज्ञकर्मणि या स्थितिः, तपसि च या स्थितिः, दाने च या स्थितिः, सा सत् इति च उच्यते विद्वद्भिः । कर्म च एव तदर्थीयं यज्ञदानतपोर्थीयम् ; अथवा, यस्य अभिधानत्रयं प्रकृतं तदर्थीयं यज्ञदानतपोर्थीयम् ईश्वरार्थीयम् इत्येतत् ; सत् इत्येव अभिधीयते । तत् एतत् यज्ञदानतपआदि कर्म असात्त्विकं विगुणमपि श्रद्धापूर्वकं ब्रह्मणः अभिधानत्रयप्रयोगेण सगुणं सात्त्विकं सम्पादितं भवति ॥ २७ ॥
In plain words
In sacrifice -- in the actual performance of sacrifice -- whatever steadiness of purpose there is, and in austerity too, whatever steadiness, and in giving too, whatever steadiness: the learned call that 'sat.' And it is also the action itself, done for that purpose -- for the sake of sacrifice, gift, and austerity. Or, put another way: it is that action which is done for the sake of the very thing this threefold name points to -- for sacrifice, gift, and austerity, that is, for the Lord's own sake -- that is what is meant; it gets called precisely by the word 'sat.' And that very action -- sacrifice, gift, austerity, whatever it is -- even if it is not sattvic on its own, even if it is flawed, but done with faith -- through the use of Brahman's threefold name, it is given real quality, made sattvic, brought to completion.
Close to the Sanskrit
'In sacrifice,' i.e. in sacrificial action, whatever steadiness [there is], and in austerity too, whatever steadiness, and in gift too, whatever steadiness -- that is called 'sat' by the learned. And action itself, done for the sake of that -- for the sake of sacrifice, gift, and austerity; or else, [it is] that of which the threefold designation is under discussion, done for the sake of that -- for the sake of sacrifice, gift, and austerity, [meaning] for the sake of the Lord -- this [is the meaning]; it is designated precisely by the word 'sat.' That very action -- sacrifice, gift, austerity, and so on -- non-sattvic, even deficient in quality, [but performed] with faith -- through the employment of the threefold designation of Brahman, becomes possessed of quality, sattvic, accomplished.