A relational symbol denoting non-parallel objects or lines in geometry and linear algebra.
Examples
Basic statement that two lines are not parallel
l_1 \nparallel l_2
Geometric proof statement
\text{If } AB \nparallel CD \text{, then the lines intersect at some point P}
Vector relationship
\vec{u} \nparallel \vec{v} \implies \vec{u} \times \vec{v} \neq \vec{0}