A PPC advertising auction bidding technique in which you scan the list of paid placement results, looking for significant differences between bids. You then adjust your bid to be just higher than the lower bid in the gap. If the first result is bidding 75 cents, the second result is bidding 45 cents, and the third result is bidding 40 cents, the largest gap is between the first two results, so a gap surfer would bid 46 cents to claim the second ranking.