This collection of essays on definitions, from Plato and Aristotle to modern times, assembles interesting, sometimes less widely known and controversial texts. They examine the subject from the point of view of philosophy which is essential for a theory of terminology seeking to establish the relationship between concepts and terms. These essays deal mainly with theoretical issues but they also consider the practice of defining and therefore serve as background to all manner of studies in terminology. In addition they form a useful complement to the better known discussions of definitions in lexicography.