tunic vs tabard

tunic and tabard both are nouns.

Word	Noun	Adjective	Verb	Adverb
tunic	Yes	No	No	No
tabard	Yes	No	No	No

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

tunic: any of a variety of loose fitting cloaks extending to the hips or knees
tabard: a short sleeveless outer tunic emblazoned with a coat of arms; worn by a knight over his armor or by a herald

Other hyponyms of tunic include chiton, gymslip, kameez, kirtle, surcoat.

tunic (noun)	tabard (noun)
any of a variety of loose fitting cloaks extending to the hips or knees	a short sleeveless outer tunic emblazoned with a coat of arms; worn by a knight over his armor or by a herald
an enveloping or covering membrane or layer of body tissue