To make something taste sweeter by adding sugar or another sweet substance. She sweetened her tea with a spoonful of honey.
To make something more agreeable or pleasant. He tried to sweeten the deal by offering a discount.
To make or become more pleasant or agreeable in manner or disposition. Her mood sweetened after she received the good news.
(intransitive) To become sweet or sweeter in taste or character. The cider will sweeten naturally as it ferments.
To enhance an audio or video recording by adding music, sound effects, or audience laughter so that it is more appealing. The editors sweetened the sitcom episode with a subtle laugh track.
To remove or neutralize sulfur compounds and other impurities from (petroleum products, natural gas, etc.) in order to improve their odor or quality. The refinery installed scrubbers to sweeten the natural gas before it entered the pipeline.