The only Eternal Truth which lives and prevails forever. Sat means Paar Braham Parmeshwar (God).  It means the entity which is the only Eternal Truth.  This word is the keyword in the Mool Manter beginning “Ik Oankaar Sat Naam…”  Sat therefore signifies the Nirgun Saroop of Akal Purakh.  This is the Param Jyot Puran Parkash part of Paar Braham Parmeshwar. This is the Atam Ras part of the Almighty.  It also signifies that part of the Sargun which has become Sat Saroop, meaning the one  who has become completely truthful and only sees, speaks, hears, delivers and serves the Eternal Truth.   Sat is GurBani as well, it is the divine wisdom, it’s every word is nothing but the description of Sat and by this virtue GurBani itself becomes a Sat and GurBani itself says that we should become GurBani, which also means that we should become Sat.