Honorific/familiar term for one's aunt. The kanji 伯母さん (older than parent) and 叔母さん (younger than parent) are available but the word is usually written in kana.
おばさんが手作りのケーキを持ってきてくれた。
My aunt brought a homemade cake.
夏休みに田舎のおばさんの家に遊びに行った。
During summer vacation I went to visit my aunt's house in the countryside.
おばあちゃん (with a long ā sound) means grandma or an elderly woman, while おばさん (short a) means aunt or a middle-aged woman, so the vowel length changes the meaning entirely.