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Wiktionary
tab stop

n. 1 On a typewriter, one of the horizontal locations where the carriage movement is halted by mechanical gears. 2 (context computing by extension English) A horizontal column in a text document where the cursor stops when the Tab key is pressed, used for alignment.

Wikipedia
Tab stop

A tab stop on a typewriter is a location where the carriage movement is halted by mechanical gears. Tab stops are set manually, and pressing the tab key causes the carriage to go to the next tab stop. In text editors on a computer, the same concept is implemented simplistically with automatic, fixed tab stops.

Modern word processors generalize this concept by offering tab stops that have an alignment attribute and cause the text to be automatically aligned at left, at right or center of the tab stop itself. Such tab stops are paragraph-specific properties and can be moved to a different location in any moment, or even removed.