The Vietnamese word "cá biển" refers to "sea-fish" or "salt-water fish." It is a noun used to describe fish that live in the ocean or saltwater environments, as opposed to "cá nước ngọt," which means "freshwater fish."
While "cá biển" specifically refers to sea fish, the word "cá" can have broader meanings depending on context. For example, "cá" can refer to fish in general, regardless of whether they live in saltwater or freshwater.