Large sea lobsters, mostly spiny lobsters, sold live from tanks. Steamed in the shell, or cooked Cantonese style with e-fu noodles, sometimes with cheese. Crayfish, 小龙虾, are often called 龙虾 too.
龙 (dragon) + 虾 (shrimp) - a dragon-sized shrimp, i.e. a lobster.
usage
In everyday speech, especially in eastern and central China, 龙虾 often means 小龙虾 (crayfish): the lobster festival held every summer in Xuyi, Jiangsu, is all about crayfish.