Your second cousin, IF your mother's cousin is her first cousin. AND
ANSWER
You call your mother's cousin by the same name your mother calls it, him, or her, unless you want to indicate your and it, him, or her relationship to a common ancestor. To do that, you are going to have to know who your common ancestor is and decide on a naming system such as 'counting' or 'removal '. Personally, I'd go for the removal naming system. The other just does not work as well when you got a lot of relationships to work with. Check the links for some good charts which will help one not be forced to think a lot about linage.
OR you stand in a social-cultural relationship which requires that you use another form of address. Examples of cultural override:
they are your cousin
cousin
your great aunt.
Your cousin. You are that person's second cousin.
Your mothers sisters Granddaughter is your second cousin
Mothers sister is aunt and son is cousin so daughter wouldn't that be a niece
Mothers sister is aunt and son is cousin so daughter wouldn't that be a niece
You could call them as a brother/sister or as a cousin... If you're asking for yourself then good luck with that family.
Your mothers sisters Granddaughter is your second cousin
Your second cousin because it would be your mum's cousin.
Your mother's first cousin is YOUR first cousin, once removed. The child of your mother's first cousin is YOUR second cousin.
Your cousin.