Within the field of linguistics (including historical linguistics), the term "isogloss" describes a distinctive feature of a language or dialect (see volumes such as The Cambridge Encyclopedia of the World's Ancient Languages, ed. Roger D. Woodard). Such features are of great importance for the purposes of linguistic classification. For example, a feature of the ancient Northwest Semitic languages is the following: prima w > y. Thus, within Proto-Semitic and subsequent non-Northwest Semitic languages and dialects, the root letters used to spell a word for "child" were /wld/. However, within the ancient Northwest Semitic languages, the word was spelled /yld/, that is, with w > y. This can be termed a linguistic isogloss and is an important datum in linguistic classification. Similarly, Proto-Semitic long /a/ becomes long /o/ in the Canaanite dialects of Northwest Semitic (see volumes such as W. Randall Garr, Dialect geography of Syria Palestine: 1000-586 BCE). Note that within the Aramaic languages and dialects of Northwest Semitic, the historic long /a/ is preserved. Thus, an ancient Northwest Semitic language in which historic long /a/ becomes long /o/ can be classed as part of the Canaanite branch of Northwest Semitic. Such features can be termed linguistic isoglosses and can be used as data of fundamental importance for the purposes of linguistic classification
Just as there are distinguishing features of related languages, there are also distinguishing features of related scripts (for a discussion of writing systems, see The World's Writing Systems, eds. Peter Daniels and William Bright). For example, a distinguishing feature of the ancient Old Hebrew script (i.e., Iron Age Old Hebrew script) is the fact that the letters bet, dalet, 'ayin, and resh do not have an open head (compare Aramaic of the same period, with its open-headed forms). Similarly, the bet of Old Hebrew has a distinctive stance (namely, leans to the right), while the bet of the Aramaic and Phoenician script series has a different stance (namely, both of these lean to the left). Recently, Christopher Rollston has suggested using the term "isograph" to designate a feature of the script that distinguishes it from a related script series (i.e., a feature that distinguishes the script of Old Hebrew, from Old Aramaic and Phoenician, etc.). That is, he proposes for it to be used as a technical term for a distinctive (and distinguishing) aspect of a script series that distinguishes it from a related script series. See his discussion in the Bulletin of the American Schools of Oriental Research 344 (2006).