"leaven" in the noun sense
1. leaven, leavening
a substance used to produce fermentation in dough or a liquid
2. leaven, leavening
an influence that works subtly to lighten or modify something
"his sermons benefited from a leavening of humor"
"leaven" in the verb sense
1. raise, leaven, prove
cause to puff up with a leaven
Source: WordNet® (An amazing lexical database of English)
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