My aunt's granddaughter is, of course, the child of one of my first cousins. This makes her my first cousin once-removed. A second cousin would be someone with whom you share a great-grandparent instead of a grandparent. She would be the child of one of your grandparents' siblings. My grandfather's niece or nephew (my parent's first cousin) is my second cousin, and her children are my 2nd cousins once-removed.
Your aunt's daughter would be your cousin, her daughter, (your aunt's granddaughter) is your second cousin.
Not your second cousin but first cousin once removed