CSA is short for 'community supported agriculture.'
According to LocalHarvest, this is a label describing a type of farm “wherein a farmer offers a given number of shares to a community, typically in the spring when the farm’s cash flow needs are the highest. Members purchase a share up front, and in exchange receive a box of vegetables each week throughout the growing season.”
Even though Community Supported Ag is a descriptor, it’s often used as a noun: “I just joined the coolest CSA; it sells eggs!”
From "Eat Where You Live", Copyright © 2008 by Lou Bendrick.