Book excerpt: In the Dictionary of Physics , central ideas and concepts are carefully introduced and explained. Each entry begins with a clear, one-sentence definition and is followed by an explanation and, where appropriate, by specific examples. The more important and more complex entries are supported by diagrams and by extra explanatory material. If relevant, the initial definition is followed by a word equation, a definition of the unit, and the symbols for the quantity and its unit. The aim of the Dictionary of Physics is to offer a focused account of the subject without leaving out any of the essential steps towards a new concept. Entries contain cross-references in italic to other entries; these further entries either support or extend the ideas of the original entry.