Search engine algorithm updates are periodic improvements made to the procedures, or algorithms, used to rank websites in natural search results.
Historically, Google has defined an algorithm update as the addition of a new ranking signal to their algorithm. Over the past few years, however, many of those signals have acquired "dials", which can be turned up and turned down, blurring the lines between subtle changes and true updates. Each year, Google makes hundreds of small improvements to its ranking algorithm—relatively few of which have any real noticeable effect on natural search results.
Not all updates involve changes to search engine ranking signals. Google makes continuous updates to its index as it crawls the Web for content, making new results available to searches. Most of Google's index updates are automated; however, occasionally are tied to a specific part of the algorithm. The specialized—manually triggered—index updates are known as data refreshes; run to remove low quality sites from search results.
Noticeable changes in search results always sparks widespread speculation and forum discussion, with assumed updates often named prior to confirmation from Google. Google updates have traditionally been named by Brett Tabke, founder and owner of Webmaster World and the PubCon conference.
Google periodically releases official statements that outline recent algorithm changes, particularly when an update causes significant fluctuations in search results.