婶 is a bound form (b.f.) — it rarely appears alone; you'll usually hear it in compounds like 婶婶, 大婶, or 婶子. It specifically refers to the wife of one's father's younger brother (a paternal aunt by marriage), but in everyday speech it's also used as a polite way to address a woman around one's mother's age, similar to 'auntie' in English.