Time is a different type of dimension than the spatial dimensions. If you're really going to bring time into it, you need to use a dimensional signature, wherein one number represents dimensions that square to a positive number, another represents dimensions that square to a negative number, and another represents dimensions that square to zero.
A video is either {2,1,0}-D or {1,2,0}-D, depending on the mathematical sign convention you chose for your spacelike dimensions.
When people use a single number, they invariably mean spacelike dimensions, as null dimensions and multiple timelike dimensions tend to give people the screaming jibblies when they try to visualize them.
The parent said "A 2D slice of a 3D object."
And that's wrong, it's a 2D projection of a 3D object.
If you include time then it's a slice in the time component, right, but that wasn't being discussed.