A doublet is a pair of words, or one of such a pair, that developed separately from the same source. Due to its mixed origins, English is particularly rich in doublets, and while the more recent Norse egg and sister survived over the older Old English forms ey and sweoster, in many cases both forms survived, albeit with slightly different meanings. Thus, we have the Old English shirt coexisting perfectly happily with the Old Norse skirt. Or ill and sick, ditch or dike. Such coexistence had, of course its ups and downs and/or regional differences: the Old English church became standard, while the Old Norse form, kirk was "relegated" to regional use.