Obachan was the nickname Keiko Ishikawa gave to her grandmother.
When Keiko was 8, she used to help her obachan create beautiful pictures of Japanese brush writing and calligraphy and was responsible for bringing fresh water for the ink brush. Keiko had forgotten the memory, but Tarmin from the Ullian delegation of telepaths helped her resurrect 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.)
One draft of the script of the episode spelled the term as "Obaachan", with a pronunciation guide of "oh-BAH-chon (second syllable 2.8 times longer than the others)", while the final draft spelled it as "Obachan". [1]