1. A system made up of n families of curves on an n-dimensional manifold in an n + l dimensional euclidean space, such that exactly one curve from each family passes through every point in the manifold, and, at each point, the tangents to the n curves that pass through that point are mutually perpendicular.

2. A set of real-valued functions, the inner products of any two of which vanish. Also known as orthogonal family.