definition of a theory

But for scientists, a theory has nearly the opposite meaning. A theory is a well-substantiated explanation of an aspect of the natural world that can incorporate laws, hypotheses and facts. The theory of gravitation, for instance, explains why apples fall from trees and astronauts float in space. Similarly, the theory of evolution explains why so many plants and animals–some very similar and some very different–exist on Earth now and in the past, as revealed by the fossil record.
Part of the Darwin exhibition.

This is a revised version of a paper which I originally read at the Department of Philosophy at Stanford University in February 1978. At that time (from January to June 1978) I was invited as a guest-professor by the chairman (it was Joe Lambert) of the Department of Philosophy at the University of California at Irvine. I was then the 3 rd exchange candidate of our exchange program between the Department of Philosophy at Irvine and ours at the University of Salzburg which we (Joe and me) founded in 1973 and which started in 1975. Since that time many scholars of both departments benefitted a lot from that exchange program. I want therefore to take the opportunity to say my warmest thanks to my friend Joe for the many troubles and for his permanent care for this important program. It is a great pleasure to me to dedicate an essay to him which I have prepared during my stay in Irvine.
The purpose of the paper is to show (1) that Aristotle had a kind of criterion of non-creativity in his theory of definition and (2) that he had a requirement for scientific explanations which amounts to finding an interpolation-sentence. The relevant passages can be found in the Posterior Analytics, second book, especially chapters 2, 7, 8, 13.

Some errors in systematic theory have arisen by attempts to define things instead of words. The abstract concept of the biological species definition is discussed, together with objections to it. The author shows that “homology” is a theoretical term and that phylogenetic definitions of it are not circular.
Michael T. Ghiselin, An Application of the Theory of Definitions to Systematic Principles, Systematic Biology, Volume 15, Issue 2, June 1966, Pages 127–130, https://doi.org/10.2307/sysbio/15.2.127

References:

http://link.springer.com/chapter/10.1007/978-94-011-3244-2_14
http://academic.oup.com/sysbio/article-pdf/15/2/127/4672333/15-2-127.pdf
http://idioms.thefreedictionary.com/in+theory