What is a galactagogue?

Alfalfa
Anise
Barley 
Blessed Thistle
Blue Vervain 
Fennel 
Fenugreek
Garlic
Ginger
Goats Rue
Hops
Nettle
Oatstraw
Marshmallow Root 
Milk Thistle
Moringa 
Shatavari
Spirulina