Sthitha Prajna is the concept, discussed in the Sankhya Yoga of the Gita, of a free being who lives in this world and appears to be like anybody else. A Sthitha Prajna is one whose mind has become absolutely still, quietened, tranquil.
When all desires of the heart have rested and heart finds joyous satisfaction in itself because it has found that all joy exists within independent of external factors, then one is spoken of as a person of steady wisdom, a Sthitha Prajna.
A Sthitha Prajna does what is necessary for the betterment of humanity with a completely tranquil mind, unperturbed. A person whose mind is tranquil will not react to situations in a way that causes harm to others because he sees everybody as his own self.