The orange-yellow root of a ginger relative, sold mostly dried and ground, with an earthy, slightly bitter taste. It gives curry powder its yellow colour and is used mainly in curries and Southeast Asian dishes.
Two senses: the spice / herb (used in curry, TCM as a blood-moving herb), and the color it gives - a warm golden yellow often written 姜黄色 in fashion and design contexts. Don't confuse with 生姜 (fresh ginger root).