Split N nodes into two groups of N/2 nodes such that the bandwidth between these two groups is minimum: that is the bisection bandwidth.
References:
[1] http://www.cs.utah.edu/~rajeev/cs6810/pres/07-6810-23.pdf
[2] http://courses.cs.washington.edu/courses/csep524/99wi/lectures/lecture7/sld006.htm