sweeten

from The American Heritage® Dictionary of the English Language, 5th Edition.

• intransitive verb To make sweet or sweeter by adding sugar, honey, saccharin, or another sweet substance.
• intransitive verb To make more pleasant or agreeable.
• intransitive verb To soften or soothe.
• intransitive verb To make bearable; alleviate.
• intransitive verb To increase the value of (collateral for a loan) by adding more securities.
• intransitive verb To enhance the attractiveness or financial desirability of (an offer, for example).
• intransitive verb Games To increase the value of (an unwon poker pot) by adding stakes before reopening.
• intransitive verb To make less acidic.
• intransitive verb To remove sulfur compounds from (fuel oil or gas).
• intransitive verb To become sweet.

from The Century Dictionary.

• To wash (gold) with hot water after the parting process.
• See the extract.
• To become sweet, in any sense.
• To make sweet to any of the senses.
• To make pleasing or grateful to the mind: as, to sweeten life; to sweeten friendship.
• To make mild or kind; soften.
• To make less painful or laborious; lighten.
• To increase the agreeable qualities of; also, to render less disagreeable or harsh: as, to sweeten the joys or pleasures of life.
• To make pure and wholesome by destroying noxious or offensive matter; bring back to a state of purity or freshness; free from taint: as, to sweeten apartments that have been infected; to sweeten the air; to sweeten water.
• To make mellow and fertile: as, to dry and sweeten soils.

from the GNU version of the Collaborative International Dictionary of English.

• intransitive verb To become sweet.
• transitive verb To make sweet to the taste.
• transitive verb To make pleasing or grateful to the mind or feelings.
• transitive verb To make mild or kind; to soften.
• transitive verb To make less painful or laborious; to relieve.
• transitive verb To soften to the eye; to make delicate.
• transitive verb To make pure and salubrious by destroying noxious matter
• transitive verb To make warm and fertile; -- opposed to sour.
• transitive verb To restore to purity; to free from taint.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb transitive To make sweet to the taste.
• verb transitive To make pleasing or grateful to the mind or feelings.
• verb transitive To make mild or kind; to soften.
• verb transitive To make less painful or laborious; to relieve.
• verb transitive To soften to the eye; to make delicate.
• verb transitive To make pure and salubrious by destroying noxious matter.
• verb transitive To make warm and fertile.
• verb transitive To restore to purity; to free from taint.
• verb transitive To make more attractive; said of offers in negotiations.
• verb intransitive To become sweet.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb make sweeter in taste
• verb make sweeter, more pleasant, or more agreeable