In Japanse, "Obasan" is a respectful name for "aunt".  The title refers to the main character Naomi's aunt, Ayako Nakano, who is the spiritual center of the book.  Obasan, who embodies the Japanese traditions of silent suffering and endurance, becomes a mother figure to Naomi in the absence of her own mother.  Although her silence keeps her at a distance, she holds the key to many of the secrets of Naomi's childhood, revealing them to her in broken English and in pictures, and Naomi takes comfort from her constancy and peace during her quest to reconcile her troubled past.

Naomi describes what Obasan means to her near the end of Chapter 3:

"The language of her grief is silence...she is every old woman in every hamlet in the world...everywhere the old woman stands as the true and righful owner of the earth.  She is the bearer of keys to unknown doorways and to a network of astonishing tunnels.  She is the possessor of life's infinite personal details".