What is the definition of a hairline?
Is it the thin strokes in a letter like "O"?
Or is it the thin strokes in a letter like "M"?
"Hairline" generally refers to a stroke or line smaller then 0.25pt in width. Sometimes it may mean smaller than 0.5pt in width.
"Hairline" is not directly associated with any type glyph and is not a term used exclusively for typography. A Hairline can be any line, any where.