adjective
An adjective is a word that describes or modifies a noun or pronoun. In this case, 'unsweet' is an adjective that describes something that is not sweet.
The unsweetened tea is a popular choice for those looking to avoid added sugars.
I prefer my coffee unsweet, just black with no sugar.
I can't stand anything unsweet, I need my desserts to be sugary and sweet.
Her words were unsweet, leaving a bitter taste in my mouth.