Meaning of your mom’s sister’s daughter

The term “cousin” can be used to refer to any daughter or son of a relative on your mother’s side in English. That is to say that your mother’s sister (younger or older) is your first cousin.

Some people might simply call her your “auntie’s daughter” because your mother’s sister is your aunt. While others might use formal terms like “cousinage”.

Cousinship is the only relationship known by law to be a blood relationship. The terms aunt and uncle are not considered to be blood relationships and are usually the terms that can also be used for non-related people.

Just like your mother’s sister’s husband (or aunt’s husband) can be referred to as your uncle even though you are not blood-related.

Mom’s sister (elder or younger) = Aunt

Mom’s sister’s daughter = Aunt’s daughter

Mum’s sister child = Aunt’s child

Aunt’s child (son or daughter) = First cousin

What is my mother’s sister’s daughter’s child called?

The answer is she is your first cousin once removed.

Mom’s sister’s daughter’s child = Aunt’s daughter’s child

Aunt’s daughter’s child = Mother’s sister’s grandchild

Mother’s sister grand child = Cousin’s child

Cousin’s child = First cousin once removed

A mother’s sister’s husband

Your mother’s sister’s husband is referred to as your ‘Uncle’.

Your mother’s sister’s son

Your mother’s sister’s husband is referred to as your ‘first cousin’.

A mother’s sister’s grandchildren

Your mother’s sister’s grandchildren are referred to as your “first cousins once removed”.

First cousin once removed

A first cousin once removed is someone who shares your first great-grandparent as a common ancestor. This is someone who is more distantly related to you than your cousin, but less distantly related than your parent or sibling.

Second cousin once removed

A second cousin once removed is someone who shares your second great-grandparent as a common ancestor. This is someone who is more distantly related to you than your cousin, but less distantly related than your parent or sibling.

Half-cousin

Half-cousins are children of your parent’s half-siblings. A half-cousin is someone with whom you share a grandparent, great-grandparent, or great-great-grandparent.

Third cousin

A third cousin is someone who shares your third great-grandparent as a common ancestor.

Eighth cousin

An eighth cousin is someone who shares your eighth great-grandparent as a common ancestor. You could be an eighth cousin to more than one person. 

Whether is a second, fourth, fifth, sixth, seventh cousin and so forth, it means you share your second, fourth, fifth, sixth, or seventh great-grandparent as a common ancestor with them.

Conclusion

I would refer to your aunt’s child as your first cousin.  It is the closest relative to you, after your parents. 

An aunt and uncle are your parents’ brother and sister, but their children are your first cousins.  If they are parents and have only one child, that child is your first cousin once removed.