A stretch mark.
A stripe, usually one of a set of parallel stripes.
One of the fillets between the flutes of columns, etc..
A line placed underneath a piece of text in order to provide emphasis or to indicate that it should be viewed in italics or (in electronic documents) that it acts as a hyperlink.
The character _.
To emphasise or stress something
To draw a line underneath something, especially to add emphasis; to underscore
Passing under a railway line.