What do I call my mother's aunt?

You call your mother's aunt your great aunt. When referring to the aunt, her name is usually simply preceded by the title, as in "Aunt Mary."

Within some families, the variation "grand aunt" is used instead of "great aunt." A person with a great aunt is considered to be a "great niece" or "great nephew" to that relative. Great aunts and great uncles are the siblings of one's grandparents. When adding an additional generation, great-grand aunts and great-grand uncles are the siblings of one's great-grandparents. As common as the word "great" is when defining relationships, it is never used to describe relationships among distant cousins.