In mathematics and mathematical physics, a coordinate basis or holonomic basis for a differentiable manifold M is a set of basis vector fields {eα} defined at every point P of a region of the manifold as
where s is the infinitesimal displacement vector between the point P and a nearby point Q whose coordinate separation from P is δxα along the coordinate curve xα (i.e. the curve on the manifold through P for which the coordinate xα varies and all other coordinates are constant).
It is possible to make an association between such a basis and directional derivative operators. Given a parameterized curve C on the manifold defined by xα(λ) with the tangent vector u = uαeα, where uα = dxα/dλ, and a function f(xα) defined in a neighbourhood of C, the variation of f along C can be written as
Since we have that u = uαeα, the identification is often made between a coordinate basis vector eα and the partial derivative operator ∂/∂xα, under the interpretation of all vector relations as equalities between operators acting on scalar quantities.
A local condition for a basis {ek} to be holonomic is (with this interpretation) that all mutual Lie derivatives vanish:
A basis that is not holonomic is called a non-holonomic or non-coordinate basis.
It is generally impossible to find a coordinate basis that is also orthonomal in any open region U of a manifold M, with an obvious exception of the real coordinate space Rn considered as a manifold with the Euclidean metric δij at every point.