This term refers to the apparent preference of positively charged nitrogen substituents for the equatorial conformation beyond what normal steric interactions would predict in rings containing an electronegative atom, such as oxygen. Substituents containing carbons with partial positive charges are not seen to exhibit the same effect.  Theoretical explanations for the reverse anomeric effect include an electrostatic explanation and the delocalization of the sp electrons of the anomeric carbon and oxygen lone pair.  There is some debate as to whether or not this is a real phenomenon.  The nitrogen containing substituents it has been reported with are quite bulky, making it hard to separate the normal effects of steric bulk and the reverse anomeric effect, if it does exist.  For example, in the molecule shown below, the pyridinium substituent strongly prefers the equatorial position, as steric factors would predict, but actually shows a stronger preference for this conformation than predicted, suggesting the reverse anomeric effect is contributing.