obasan おばさん

In Japanese, obasan おばさん means "aunt," but sometimes it refers to an older woman.

Not to be confused with obaasan おばあさん, or obāsan with a macron, which means "grandmother" instead.

Kanji

The word obasan can be written with kanji in multiple ways, depending on the meaning of the word:

obasan
伯母さん
Aunt, who is your parent's older sister. In other words, the oneesan of your otousan or okaasan.
obasan
叔母さん
Aunt, who is your parent's younger sister. In other words, the imouto of your otousan or okaasan.
obasan
小母さん
Elder woman.

Variants

With family words in o__san お〇〇さん pattern, the o~ お～ prefix can be removed, and the honorific suffix can be changed between ~san ～さん, ~chan ～ちゃん, and ~sama ～さま.

obachan
おばちゃん
(chummier.)
bachan
ばちゃん
(same meaning.)
obasama
おばさま
(more respectful.)

vs. 叔母

The difference between obaasan and oba 伯母 (叔母) is that oba doesn't have honorifics.

Since you don't use honorifics toward yourself, when talking to other people about your family, you use oba 叔母 to refer to your aunt, and obasan 叔母さん to refer to their aunt.

However, when talking to your own family, obasan refers to your own aunt.

Toward Older Women

The word obasan 小母さん can also refer not to your "aunt" but to an elder woman instead. There's no hard rule about how old she has to be. It's a mid-point between oneesan and obaasan.

In anime, sometimes teenager girls and young women get called obasan by children and get real mad because of it, often correcting the child saying they're an oneesan instead.

When the word is used like this, it sometimes comes after the demonstrative pronouns kono, sono, ano.