Parallax is a related almost opposite) word that ha the same double l .
Must people know the definition of parallel but here it is again. Basically, if two lines are parallel, they are equal distance apart everywhere. They run side-by-side and never intersect.
Parallax is a bit more complicated. In general, it mean an apparent change in the position of an object resulting from a change in position of the observer. A good example is two people in different location in the world viewing a star.
I first heard of parallax in the context of viewing a gauge or dial. If you stand directly in front of it, you get a true reading. If you move a foot or two to the side, you get a different and erroneous, reading.