Liǎng ge fǎxiàngliàng píngxíng shuōmíng liǎng píngmiàn píngxíng.
If two normal vectors are parallel, the two planes are parallel.
Tips
memory
法 here is short for 法线 ('normal line / perpendicular'), not 'law'. So 法向量 = 'perpendicular vector' = the normal vector. Same 法 as in 法线 (normal line), distinct from 法律 (law).
usage
Standard term in high-school and university math (linear algebra, multivariable calculus, computer graphics shading). Pair with 切向量 ('tangent vector').
