Uncle / Aunt (mother's brother and his wife)

Literally

Maternal uncle / his wife

When to use it

Used specifically for the mother's brother; mami is his wife. Mama is also a very common, friendly way to address a rickshaw-puller, a tea-stall owner or a shopkeeper, especially among students and younger people. Mami is not used for strangers that way.

How it feels

Warm and casual; widely used beyond blood relatives as a friendly form of address.

Formality

Familiar but respectful