Obachan

From Memory Alpha, the free Star Trek reference.

Obachan is the nickname Keiko Ishikawa gave to her grandmother. When Keiko was 8, she used to help her Obachan create beautiful pictures of Japanese Calligraphy. Keiko had forgotten the memory, but Tarmin from the Ullian delegation of telepaths helped her ressurect the forgotten memory. (TNG: "Violations")

Obachan is a Japanese word for "grandmother." The word is usually written as Obaachan to distinguish it from the word for "aunt." They sound almost exactly the same save the longer "a" sound for grandmother. The -chan suffix indicates familiarity and a casualness to the relationship. Grandma may also be seen as "Obaasama" (very polite and formal) or "Obaasan" (polite standard address.)