BM = 0
Contra-flexure = contra (opposite) + flexure (bending) 
We call point of contra-flexure at some location of any bending member like- beam where bending moment changes its sign along the length of the member for a particular load case. So, in Bending Moment Diagram, bending moment curve crosses the zero line at contra-flexure or simply, bending moment is zero at point of contra-flexure. It occurs in overhanging beam. In other beams also it can occur but in overhanging it will definitely will be there.