Tabs in a GUI window. Use the whitespace, text-formatting, or CSV tags if your question involves the tabulation (tab) character.
A tab is a panel in a window, tyipcally chosen via a row of labels at the top (or sometimes a column of labels on the side). For example, many web browsers support multiple tabs in the same window, each showing one web page and having its own history.
For tab characters, see tabulation.