filler vs stiffener

filler and stiffener both are nouns.

Word	Noun	Adjective	Verb	Adverb
filler	Yes	No	No	No
stiffener	Yes	No	No	No

As nouns, stiffener is a hyponym of filler; that is, stiffener is a word with a more specific, narrower meaning than filler:

Other hyponyms of filler include putty, mastic, size, sizing.

filler (noun)	stiffener (noun)
the tobacco used to form the core of a cigar	a member attached to a beam web to prevent loss of strength due to web buckling.
anything added to fill out a whole	material used for stiffening something
copy to fill space between more important articles in the layout of a magazine or newspaper
100 filler equal 1 forint in Hungary
used for filling cracks or holes in a surface