Identical, one superimposed on the other. Two or more geometric figures that share all points. For example, two coincident lines would look like one line since one is on top of the other.
When two drivers are constructed such that their acoustical centers appear to be in the same location in space at their crossover frequencies, they are said to be coincident.
Coincident is a geometric term that pertains to the relationship between two vectors. Vectors consist of a "magnitude" and a "direction". Vectors can be said to be coincident when their direction is the same though the magnitude may be different.