Science (from Latin scientia, meaning "knowledge") is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about nature and the universe. In an older and closely related meaning, "science" also refers to a body of knowledge itself, of the type that can be rationally explained and reliably applied. Ever since classical antiquity, science as a type of knowledge has been closely linked to philosophy. In the West during the early modern period the words "science" and "philosophy of nature" were sometimes used interchangeably, and until the 19th century natural philosophy (which is today called "natural science") was considered a separate branch of philosophy in the West.In modern usage, "science" most often refers to a way of pursuing knowledge, not only the knowledge itself. It is also often restricted to those branches of study that seek to explain the phenomena of the material universe. In the 17th and 18th centuries scientists increasingly sought to formulate knowledge in terms of laws of nature. Over the course of the 19th century, the word "science" became increasingly associated with the scientific method itself, as a disciplined way to study the natural world, including physics, chemistry, geology and biology. It is in the 19th century also that the term scientist began to be applied to those who sought knowledge and understanding of nature. However, "science" has also continued to be used in a broad sense to denote reliable and teachable knowledge about a topic, as reflected in modern terms like library science or computer science. This is also reflected in the names of some areas of academic study such as social science and political science.