Refers to the frequent, almost daily updating of organic search results at Google. Everflux occurs between major updates when individual pages are updated, added, or removed from the index.
Basically Google updates its entire index of web sites once a month (known as the Google Dance, it lasts for a couple of days). Google results also change throughout the month, in between the major updates; this continuous change became known as everflux. It happens because Google, in an ongoing effort to make their results as relevant as possible, has introduced a crawl that runs each day. The idea behind the "fresh crawl" is to update pages that change regularly, allowing Google to serve results that are up-to-date with current events.