Interception is defined as follows: one (or two? rarely three) pair of signs in the chart does not appear on any house cusp, but instead is contained within houses that are larger than 30 degrees. Of course, this can only occur when one uses unequal house systems, since equal house systems have houses of 30 degrees.
http://www.ncgr-turkey.com/interception?_escaped_fragment_=interception/crb4#!interception/crb4