The largest value within a given range, but not necessarily the absolute largest value within the entire set. – Wikipedia
When you’ve been doing a lot of iterative testing and are starting to see diminishing returns, you may have hit a local maximum for your optimization efforts on that area of the site. It may be time to test something bigger and more transformative, or change focus to a different area of the site. A good technique to avoid optimizing to local maxima would be to incorporate some kind of iterative-transformative ranking to your test ideas and prioritizing on a ratio; something like “we’ll do one big transformative test for every ten little iterative tests”.