Grandma is the most popular name for a grandmother. Grandma is also the most traditional moniker for female grandparents, as it is easy to pronounce and among the least awkward of nicknames.
Interestingly enough, Grandma is not just popular in the United States. Over 20 countries have a word for Grandma. Part of the appeal may be that Grandma serves as a middle ground between formal and informal titles. It is not as harsh sounding as Grandmother, yet does not cross into the juvenile sounding territory of Mimi, GaGa and Gammy. However, many grandparents admit the name choice is simply due to a lack of creativity.