scalar product

(Also called dot product, direct product, inner product.) A scalar equal to the product of the magnitudes of any two vectors and the cosine of the angle θ between their positive directions. For two vectors A and B, the scalar product is most commonly written A · B, read “A dot B,” and occasionally as (AB). If the vectors A and B have the components A_x, B_x, A_y, B_y, and A_z, B_z along rectangular Cartesian x, y, and z axes, respectively, then

If a scalar product is zero, one of the vectors is zero or else the two are perpendicular.
See vector product.