Autocomplete, also known as autosuggest or search suggest, is a feature that provides predictions as you type in a text box. It is commonly associated with search engines, though it may be used for other purposes as well.
One of the most common places you will see autocomplete is in the address bar of your web browser. Since most browsers allow you to perform searches directly from the address bar, you will likely see search suggestions appear as you begin typing. However, since you can also type URLs in the address bar, your browser may also suggest webpages from your browsing history or bookmarks, or other popular websites.
If you visit a search engine website, like Google or Bing, you'll most likely be presented with a list of search suggestions immediately as you start typing in the search box. These suggestions come primarily from a history of aggregate users searches recorded by the search engine. However modern autocomplete algorithms may use other information as well. For example, a search engine may use your location to provide relevant queries to your surroundings. It may take into account the current time or day to provide suggestions relevant to the time of your search. If you are logged in or allow cookies to store your browsing information, the search engine may use your browsing or search history to provide more targeted search suggestions.
While autocomplete is a popular feature for web search, many other websites use it as well. For example, websites like Amazon and Best Buy provide suggestions as you search, which are relevant to your browsing or purchase history. Even this website, TechTerms.com, uses autocomplete to provide a list of commonly searched tech terms as you type. You can try it for yourself by typing in the search field at the top of the page!
NOTE: The boxes that display word suggestions as you type in Android and iOS are similar to autocomplete, but they are used for more general purposes, such as texting and composing documents. In Android, this feature is called predictive text, while in iOS, it is called QuickType.