महात्मा

Mahatma means a great soul, a person whose heart has grown large in the Lord's shelter. In the ninth chapter the great souls are those who take shelter in the divine nature and worship the Lord with an undivided mind, knowing him as the imperishable origin of all beings (Gita 9.13). The next verse names what they do: glorifying, striving with firm resolve, bowing, and being ever joined (Gita 9.14).

In the seventh chapter such a great soul, who after many births surrenders to the Lord knowing that all is Vasudeva, is called very rare (Gita 7.19). In the eighth, the great souls who have reached the highest perfection do not take birth again after reaching him (Gita 8.15).

Common misreading: that a great soul is someone of special dress or rank.

Better sense: the verses do not identify a great soul by dress or office. He is known by where he has taken shelter and where his mind rests. The Lord's promise in the ninth chapter, to carry the care of undivided devotees (Gita 9.22), is tied to that same undivided mind.

The whole topic is in The Steadfast Devotee and Divine Care.