姨妈是指你的直系亲属，而阿姨是指和妈妈年龄差不多的女性的称呼。

姨妈 can only be used in family. It refers to the older sister of your mother. Suppose your mother has 4 sisters and she is the third daughter in the family, generally, you should call the first daughter "大姨妈", the second daughter "二姨妈", the fourth daughter "四姨", the fifth daughter "五姨" or "小姨". You can also bring her first name into the appellation, for example, one of my 姨's first name is 小平, I call her "小平姨". It all depends on your preference. 阿姨 can be used in calling any female that is older than you by 10- 20 years, OR, the female that is under 50 years old. In family, you can only call your mother's YOUNGER sister "阿姨".

Generally speaking,"姨妈"or called "姨娘"meaning that she is my mother 's sisters ,but the relationships also can be called "阿姨"in some regions ."阿姨"is most universal applied to describing the relations of my mother 's friends,or also can be used when you talk with a stranger women ,it is a politely manner.

姨妈 is mother's old sister 阿姨 is mother's young sister or friend

Today,Chinese usually call the sisters of mother "姨妈” and call the acquaintances or friends of mother "阿姨".