Cognate

A cognate is a form in a language which corresponds to a form in another language (or a different variety of the same language, or even to a different form within the same language/variety) due to a historical/genealogical relationship between the two forms.

Two cognate forms can be traced back to one common proto-form. In most cases the two forms will have undergone sound and/or meaning changes and will therefore no longer be identical in their phonetic form and/or meaning.

Comments
By comparing a set of cognates in two related languages regular sound correspondences can be established with the comparative method. In this way sound changes like the ones described by Grimm's Law can be recognized.

Historical continuity between the forms compared is a necessary condition to be considered cognate. Yet, several factors lead to lack in continuity. Fox (1995: 62f.) lists replacement of words and remodeling via analogy as the main factors for this. He further notes that replacement of a word by a loan from a related language or dialect as well as similarity of forms due to "universal processes of invention" (such as onomatopoeia) can often lead to a wrong assumption of continuity and/or relatedness.

Reference

 * Fox, Anthony. 1995. Linguistic Reconstruction. An Introduction to Theory and Method. Oxford: Oxford University Press.