Answer: skew lines
An example:
Look where two walls meet. They meet at a vertical line. Let's call that line A.
Now look at where the ceiling and another wall meet (this wall is not the same as the previous two walls mentioned). The ceiling and this new wall meet at line B
Lines A and B are skew lines since they'll never cross even if you extend them out forever. They aren't parallel lines either because they don't point in the same 3D direction.
Check out the diagram below.