The active ingredient in asprin
and one of the ingredients in Alka-Seltzer
. Acetylsalicylic acid (or its various translations) may be used as a generic term in countries where Bayer's trademark claim to asprin was not invalidated after World War I (e.g., Germany, Canada, Mexico).
The term may also see use where it is desired to refer to the pure chemical, as opposed to any medical formulations. It may also see use where the person saying it wants to sound scientific or very intelligent.
I took some acetylsalicylic acid for my headache.