science is knowledge of the natural world obtained through systematic observation, experiment, and reasoning — and, in a broader and older sense, any organized body of knowledge or a particular branch of study.
The narrow modern sense is a late arrival. For most of the word's life in English it meant knowledge generally, then book-learning, then a discipline: the medieval curriculum spoke of the seven liberal sciences, and theology was long counted among them. The restriction to the study of the material universe and its laws emerges around 1725 and becomes predominant only in the mid-nineteenth century. The word scientist is younger than the practice it names, coined by the English polymath William Whewell in the 1830s because natural philosopher had ceased to fit the people doing the work.
What separates science from other organized knowledge is procedural rather than topical. A claim enters science by being stated so that evidence could count against it, tested against observation, and reproduced by others who did not make it — which is why replication failures are treated as a crisis rather than a curiosity. Mathematics, by contrast, proceeds from axioms by proof and needs no experiment; technology applies findings without necessarily generating them; and engineering is judged by whether the thing works, not by whether the explanation is true. Thomas Kuhn's account complicated the tidy picture by arguing that normal science operates inside a governing 📝paradigm and changes not by steady accumulation but by periodic upheaval.
The word came into English in the mid-fourteenth century from Old French science, from Latin scientia, "knowledge, a knowing, expertness," from scire, "to know." Scire probably meant something sharper first — to separate one thing from another, to distinguish — placing knowing and cutting apart in the same gesture and making science a distant relative of 📝discernment. The same root gives English conscience, omniscience, prescience, and nescience.