"Sanitary napkin" is a euphemism for any of the disposable external items used to absorb menstrual discharges, usually a maxi-pad or panty liner. The place I see this phrase most often is on the boxes in stalls of women's public restrooms. Tampons can usually be flushed, while maxi-pads and panty liners can't be, so the box is there for the disposal of non-flushable items.