Well, this is going to sound strange, but currently I call her "grandma".
Many reasons -- I was raised by my grandparents (for the most part) so my true maternal grandmother (who I called grandma) was mostly my mother, for all intents. Now she passed on 30 years ago.
I began to raise my kids and take care of my mother at the same time -- so most of the time (and consistently for the past 15 years) we have lived in the same house (my mom, me, and my kids). The kids always call her grandma. They call me Mom. So it just adapted that I started calling her grandma too (their grandma) to clarify who was being addressed in the household at any point. ("Grandma likes it cooked that way." "Grandma is outside", etc) It sounds confusing, but it's not.
but it's not a choice on the list.
If I were introducing her to someone else, I would use the term "my mom" probably, so I'll vote that way. (corrected - I decided to vote the last choice)