Each of the words "evolution", "fact" and "theory" has several meanings in different contexts. "Evolution" means change over time, as in stellar evolution. In biology it refers to observed changes in organisms, to their descent from a common ancestor, and at a technical level to a change in gene frequency over time; it can also refer to explanatory theories (such as Charles Darwin's theory of natural selection) which explain the mechanisms of evolution. To a scientist, "fact" can describe a repeatable observation that all can agree on; it can refer to something that is so well established that nobody in a community disagrees with it; and it can also refer to the truth or falsity of a proposition. To the public, "theory" can mean an opinion or conjecture ("e.g.", "it's only a theory"), but among scientists it has a much stronger connotation of "well-substantiated explanation". With this number of choices, people can often talk past each other, and meanings become the subject of linguistic analysis.