x1 is aligned diagonally along/between nonadjacent vertices x2 as in polytope x3; x1 is a diagonal line segment/linear manifold of lower dimension as viewed in frame of reference x3; x1 is crooked (one sense), skew (one sense, see notes), off-kilter (one sense), away from center/off-center, non-orthogonal/not perpendicular nor parallel, at an angle, perhaps non-vertical and non-horizontal, diagonal to x2 in figure/coordinate system x3.
Not for use in: entries of tensors/matrices (confer: daigno), certain geometric meanings (such as with Cartesian products), etc. Only for purely 'visual' geometric objects/figures/frames. The polytope in question need not actually be 'drawn'; an oriented frame of reference naturally 'projects' a polytopic sense onto all objects. x1 can be any linear manifold of lower dimension than the space in which it is embedded (defined by x3). The skewness is not relative to another linear manifold in some higher-dimensional space (the usual definition of "skew" in geometry) - it is simply a skewness (in a layperson sense) relative to points in a figure or axis in a coordinate system. Proposed by Gleki.