1.  leavening(n = noun.cognition) leaven - an influence that works subtly to lighten or modify something; "his sermons benefited from a leavening of humor"
is a kind of imponderable

2.  leavening(n = noun.substance) leaven - a substance used to produce fermentation in dough or a liquid;
is a kind of substance
has particulars: baking powder, sourdough, barm, yeast

1.  The act of making light, or causing to ferment, by means of leaven. [1913 Webster]

2.  That which leavens or makes light. Bacon. [1913 Webster]