From the ancient world, starting with Aristotle, to the 19th century, the term "natural philosophy" was the common term used to describe the practice of studying nature. It was in the 19th century that the concept of "science" received its modern shape with new titles emerging such as "biology" and "biologist", "physics" and "physicist" among other technical fields and titles; institutions and communities were founded, and unprecedented applications to and interactions with other aspects of society and culture occurred. Isaac Newton's book Philosophiae Naturalis Principia Mathematica (1687), whose title translates to "Mathematical Principles of Natural Philosophy", reflects the then-current use of the words "natural philosophy", akin to "systematic study of nature". Even in the 19th century, a treatise by Lord Kelvin and Peter Guthrie Tait, which helped define much of modern physics, was titled Treatise on Natural Philosophy (1867).
In the German tradition, Naturphilosophie (philosophy of nature) persisted into the 18th and 19th century as an attempt to achieve a speculative unity of nature and spirit. Some of the greatest names in German philosophy are associated with this movement, including Goethe, Hegel and Schelling. Naturphilosophie was associated with Romanticism and a view that regarded the natural world as a kind of giant organism, as opposed to the philosophical approach of figures such as John Locke and Isaac Newton who espoused a more mechanical view of the world, regarding it as being like a machine.
The term natural philosophy preceded our current natural science (i.e. empirical science). Empirical science historically developed out of philosophy or, more specifically, natural philosophy. Natural philosophy was distinguished from the other precursor of modern science, natural history, in that natural philosophy involved reasoning and explanations about nature (and after Galileo, quantitative reasoning), whereas natural history was essentially qualitative and descriptive.
In the 14th and 15th centuries, natural philosophy was one of many branches of philosophy, but was not a specialized field of study. The first person appointed as a specialist in Natural Philosophy per se was Jacopo Zabarella, at the University of Padua in 1577.
Modern meanings of the terms science and scientists date only to the 19th century. Before that, science was a synonym for knowledge or study, in keeping with its Latin origin. The term gained its modern meaning when experimental science and the scientific method became a specialized branch of study apart from natural philosophy.