At a guess, it includes things like the tricks in http://dtic.mil/dtic/tr/fulltext/u2/a456960.pdf http://dtic.mil/dtic/tr/fulltext/u2/a456960.pdf ("DETECTION A

At a guess, it includes things like the tricks in http://dtic.mil/dtic/tr/fulltext/u2/a456960.pdf http://dtic.mil/dtic/tr/fulltext/u2/a456960.pdf ("DETECTION AND JAMMING LOW PROBABILITY OF INTERCEPT (LPI) RADARS") but (1) it's obviously not a very precise term and (2) it would be surprising if all of the relevant techniques were available in an unclassified review from 2006.