Bhagavad Gita
17.26

सद्भावे साधुभावे च सदित्येतत्प्रयुज्यते | प्रशस्ते कर्मणि तथा सच्छब्दः पार्थ युज्यते ||

sad-bhāve sādhu-bhāve ca sad ity etat prayujyate praśaste karmaṇi tathā sac-chabdaḥ pārtha yujyate

Translation

The word 'Sat' is used in the sense of reality and goodness; and also the word 'Sat' is used to designate a praiseworthy action, O Arjuna.

Interpretation

Sat — Being, Reality, Truth — is used in multiple sacred ways: to denote ultimate reality (sat-bhava — the state of being real, existing), to denote goodness and virtue (sadhu-bhava — the quality of the truly good person), and to denote praiseworthy action. When we say something is 'sat,' we are saying it participates in reality — it is genuine, true, substantial. Actions aligned with truth and goodness participate in Sat. This is why we call true people 'sat-purushas' and why truth-telling is 'sat-vachana.' Sat is the value that underlies all value.