Definition - What does Mahatma mean?
Mahatma is a Sanskrit term which means "the great soul." It is derived from maha, meaning "great" and atma, meaning "soul" or "true Self."
The term was adopted by Buddhism and Jainism, but its usage in Hinduism is more extensive. The title of Mahatma is bestowed upon those who are spiritually developed.
Yogapedia explains Mahatma
One of the most famous personalities bestowed with the title of Mahatma is Mohandas Karamchand Gandhi of India, who followed the path of ahimsa (meaning "non-violence") when he led the country to independence. The philosophy of the three wise monkeys that see no evil, hear no evil and speak no evil was adopted by Gandhi in his life and fight for freedom.
In the pure sense, Mahatma is an elevated state of the mind that can be achieved with the right thoughts and actions.