A closed interval corresponding to a finite portion of an infinite line. Line segments are generally labeled with
two letters corresponding to their endpoints, say 
and 
,
and then written 
.
The length of the line segment is indicated with a vinculum,
so the length of the line segment 
would be written 
.
Curiously, the number of points in a line segment is equal to that in an entire one-dimensional space (a line), and also to the
number of points in an 
-dimensional
space, as first recognized by Georg Cantor.
