The term 'single-file' is used to describe a formation of people or objects that are arranged in a line, one behind the other. This formation can be described using several words, such as 'TANDEM' (6 letters), 'inarow' (6 letters), 'linedup' (7 letters), 'onebyone' (8 letters), and 'oneatatime' (10 letters). All of these terms refer to the same concept of having a single line of people or objects.