A feature where a program proposes the user to complete a text input based on what the user started to type. Autocomplete proposes the most likely string input.
Questions that have the autocomplete tag should be about enabling, disabling, correcting or otherwise tweaking the autocomplete feature in browsers, search engines, word processors or programming editors.
Autocomplete is a composition of the words "automatic" and "complete". It stands for a program's attempt to automatically calculate and predict the most likely string a user wants to write in the program. The user can then select one of the proposals. The goal of autocomplete is to save time writing.
Typical applications are in word processors such as Microsoft Word (based on a list of words), in browsers where the predictions are based on the browsing history or in email clients where already used email addresses are proposed.
Example: When you are in a browser's address bar and you start typing go (for Google), you will like receive google.com as a proposal to complete your input.
The calculations are based either on past input or on a pool of likely inputs such as frequently used words or URLs. The more often a string is used, the more likely it will be proposed to autocomplete autocomplete. A string can consist of one word or of several words.
For a more thorough discussion you can refer to Wikipedia