A hole in a graph.
That is, a discontinuity that can be "repaired" by filling
in a single point. In other words, a removable discontinuity is
a point at which a graph is not connected but can be made connected
filling in a single point.
Formally, a removable discontinuity
is one at which the limit of the function exists
but does not equal the value of the function at that point;
this may be because
does not exist at that point.
Step discontinuity, essential discontinuity