etymology

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The origin and historical development of a linguistic form as shown by determining its basic elements, earliest known use, and changes in form and meaning, tracing its transmission from one language to another, identifying its cognates in other languages, and reconstructing its ancestral form where possible.
2. n. The branch of linguistics that deals with etymologies.

Century Dictionary and Cyclopedia

1. n. That part of philology which treats of the history of words in respect both to form and to meanings, tracing them back toward their origin, and setting forth and explaining the changes they have undergone.
2. n. Specifically The particular history of a word, including an account of its various forms and senses. In its widest sense, the etymology of a word includes all its variations of form and spelling, and all its different meanings and shades of meaning, from its first appearance in the language to the present time, and, further, the same facts concerning the original or the cognate forms of the word in other languages. This would be impracticable for any large number of words, and accordingly the fullest etymologies, as in this dictionary, give but one form or a few typical forms for a given period of a language, or but one form for the whole period of the language, with a like summary treatment of the meanings, a more complete exhibition of forms and meanings being given only at critical or important points in the history. In a very restricted but common acceptation, the word implies merely the “derivation” of the word, namely, the mention of the word or root from which it is derived, as when bishop is said to be “from Greek ἐπίσκοπος,” or chief “from Latin caput.”
3. n. In grammar, that division of grammar which treats of the parts of speech and their inflections.

Wiktionary

1. n. uncountable The study of the historical development of languages, particularly as manifested in individual words.
2. n. countable An account of the origin and historical development of a word.

GNU Webster's 1913

1. n. That branch of philological science which treats of the history of words, tracing out their origin, primitive significance, and changes of form and meaning.
2. n. That part of grammar which relates to the changes in the form of words in a language; inflection.

WordNet 3.0

1. n. the study of the sources and development of words
2. n. a history of a word