Denotes the sourdough starter or fermenting agent that causes bread dough to rise. In the ancient world, a small piece of old dough was reserved to leaven a new batch, making the image of hidden, spreading influence natural and powerful. Jesus warns his disciples to 'beware the leaven of the Pharisees' (Matt 16:6; Mark 8:15; Luke 12:1), using the word as a metaphor for the quiet, pervasive spread of corrupt teaching. Yet in Matt 13:33 the same image appears positively: the kingdom of heaven is like leaven a woman hid in flour until all was leavened. Paul picks up the metaphor in 1 Cor 5:6-8 and Gal 5:9, warning that a little leaven leavens the whole lump. Every major translation language renders it with a single cognate term (Spanish levadura, French levain, German Sauerteig).