An alphabetical list of the (principal) words or (less commonly) topics present in a text or texts, usually with citations of the passages in which they are found. In later use also: a generated list of instances in which a particular word occurs in a digital corpus, typically with the surrounding context also excerpted .
Concordances were originally compiled for the Bible and later, in the 19th century, for authors such as Milton, Shakespeare, Homer, and Tennyson. By the latter half of the 20th cent. concordances could be compiled relatively quickly using computational methods. (Oxford English Dictionary)