The term nuclear family eveloped in the western world to distinguish the family group consisting of parents, most commonly a father and mother, and their children, from what is known as an extended family. Nuclear families can be any size, as long as the family can support itself and there are only parents and children (or the family is an extended family.) According to Merriam-Webster the term dates back to 1947 and is therefore relatively new, although nuclear family structures themselves date back thousands of years. The term "nuclear" was used because of its original Latin meaning, "kernal" or "nut". Today roughly one quarter of households in the United States, for example, are described as consisting of nuclear families, making them the third most common household arrangement in that nation.