Coincident generally means for two things to share the same location. In GIS this is most often discussed in the context of a topology, and refers to when features share an unbroken sequence of common vertices along a common boundary. For example:
- points are coincident if they share the exact same coordinates
- lines and polygons are coincident (or partially coincident) if they share one or more edges with identical vertices
Note that coincident features may or may not overlap as the term applies to either a portion or all of their boundaries. Also, even if edges are coincident, the features are not said to be fully coincident from a topology standpoint unless they share all identical vertices along their common boundary - ie, if there are two lines, both with the same common endpoints but one has an extra vertex in the middle the other does not.
Feature coincidence is especially important when generating 100% coverage polygons to ensure that there are no gaps or overlaps (aka slivers) between any two polygons. Such gaps and overlaps can result if one of two adjoining polygons is missing a vertex present in the other, or a pair that should match don't share the same coordinates. It is also important when linear features, such as rivers, are used to delineate the boundary of areal features, such as a political jurisdiction.