leaven

plural leavens

1. countableuncountableAny agent used to make dough rise or to have a similar effect on baked goods.

   Synonyms: leavening agent, raising agent

2. countablefigurativelyuncountableAnything that induces change, especially a corrupting or vitiating change.

   The leaven of insincerity worked through all the fibres of his being.

   1926, T.E. Lawrence, Seven Pillars of Wisdom, New York: Anchor, published 1991, page 213

   2 more examples

   Take heed, beware of the leaven of the Pharisees, and of the leaven of Herod.

   1611, The Holy Bible, […] (King James Version), London: […] Robert Barker, […], →OCLC, Mark 8:15

   On the far side was reflected a single red and meteoric cloud, which had treasured one last crimson ray from the sunset, or perhaps nursed within it the fiery leaven.

   1834, L[etitia] E[lizabeth] L[andon], chapter XXXVI, in Francesca Carrara. […], volume III, London: Richard Bentley, […], (successor to Henry Colburn), →OCLC, page 299

3. JudaismcountableuncountableChametz.

   On the fourteenth day of Nisan men search for leaven by candlelight.

   1878, “On The Passover”, in Joseph Barclay, transl., The Talmud, page 95

present singular third person leavens · participle present leavening · participle past leavened · past leavened · alternative leven

1. transitiveTo add a leavening agent.

   Yeast is used to leaven bread, making it rise during baking.

2. transitiveTo cause to rise by fermentation.
3. figurativelytransitiveTo temper an action, decision, attitude, characteristic, etc.

   Duke. No more euasion: We haue with a leauen'd, and prepared choice Proceeded to you; therefore take your honors:[...]

   c. 1603–1604 (date written), William Shakespeare, “Measure for Measure”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC, [Act I, scene i]

   2 more examples

   “A tinge of Jewish blood is not a bad thing. It leavens the”—she looked at him—“stolid stupidity of the ordinary Englishman.”

   1920, Agatha Christie, The Mysterious Affair at Styles, London: Pan Books, published 1954, page 125

   With fresh material, taxonomic conclusions are leavened by recognition that the material examined reflects the site it occupied; a herbarium packet gives one only a small fraction of the data desirable for sound conclusions. Herbarium material does not, indeed, allow one to extrapolate safely: what you see is what you get[…]

   1992, Rudolf M[athias] Schuster, The Hepaticae and Anthocerotae of North America: East of the Hundredth Meridian, volume V, Chicago, Ill.: Field Museum of Natural History, →ISBN, page vii

4. figurativelytransitiveTo imbue; to infect; to vitiate.

   Her optimism seemed to leaven the otherwise tense environment.

   3 more examples

   The comedian's jokes helped leaven the atmosphere during the serious meeting.

   With these and the like deceivable doctrines, he levens also his prayer.

   1649, John Milton, Eikonoklastes, London, published 1756, page 30

   […] pursue virtue virtuously: leven not good actions, nor render virtues disputable. Stain not fair acts with foul intentions […]

   1716, Thomas Browne, edited by Samuel Johnson, Christian Morals, 2nd edition, London: J. Payne, published 1756, Part I, p. 7

5. To rise or become larger; to prove.