I don’t expect to get one definite answer here, but Google can’t give me one either. Some websites say they’re your second cousin, and others say they’re your first cousin once removed. I’m trying to figure out some family tree stuff so I need to know the difference between the (groups of) terms.
That someone needs to share both grandparents or both great grandparents. Otherwise they are you half first cousin or half second cousin, like you would have half siblings if you shared only one parent with them.
And then there are double cousins…