What is surrounded by a membrane fluid filled?

A structure that is surrounded by a membrane and filled with fluid is typically referred to as a "vesicle." Vesicles are small, membrane-bound sacs found within cells, often containing liquids or cytoplasm. They play crucial roles in various cellular processes, including transport, storage, and metabolism. Examples include lysosomes, which contain digestive enzymes, and mitochondria, which are involved in energy production.