A hammer union is a robust connection used in the oil and gas industry to link pipelines or fittings, featuring male and female subs with tapered threads secured by a nut, often hammered into place, providing a strong seal capable of withstanding high pressures and tough conditions; available in various pressure ratings, materials, and sizes, these unions are crucial for temporary or frequently changed connections in drilling, production, and other oilfield operations, requiring proper installation and maintenance to ensure safe and effective usage.