A coordinate chart is a way of expressing the points of a small neighborhood, usually on a manifold , as coordinates in Euclidean space. An example from geography is the coordinate chart given by the functions of latitude and longitude. This coordinate chart is not valid on the whole globe, since it doesn't give unique coordinates at the north or south pole (which way is east from the north pole?).

Technically, a coordinate chart is a map

where is an open set in , is an open set in and is the dimension of the manifold. Often, through notational abuse, the open set is equated with , and calculations on the manifold are done in the coordinate chart. This technique has the drawback that it must be checked whether a change of coordinates affects the result of a calculation.

The map must be one-to-one, and in fact must be a Homeomorphism. On a smooth manifold, it must be a diffeomorphism, although if the chart defines the smooth structure then this is a tautology. Similarly, on a complex manifold, the map is holomorphic.

If there are two neighborhoods and with coordinate charts and , the transition function is well-defined since coordinate charts are one-to-one.