Grass-fed refers to ruminants — mostly cattle, sometimes lamb or bison — that ate grass and forage rather than grain concentrates. The term is inconsistently regulated: USDA withdrew its official grass-fed standard in 2016, leaving the term to various private certification bodies.
Key distinction: "grass-fed" can mean the animal was raised on grass but finished on grain for the last few months (standard industry practice for many commercial programs). "100% grass-fed" or "grass-finished" means the animal never ate grain. Flavor and fat composition differ meaningfully between the two.
Reliable third-party certifications include American Grassfed Association and Certified Grassfed by AGW.