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A headword, lemma, or catchword is the word under which a set of related dictionary or encyclopaedia entries appears. The headword is used to locate the entry, and dictates its alphabetical position. Depending on the size and nature of the dictionary or encyclopedia, the entry may include alternative meanings of the word, its etymology, pronunciation and inflections, compound words or phrases that contain the headword, and encyclopedic information about the concepts represented by the word.
For example, the headword bread may contain the following (simplified) definitions:
The Academic Dictionary of Lithuanian contains around 500,000 headwords. The Oxford English Dictionary (OED) has around 273,000 headwords along with 220,000 lemmas, while Webster's Third New International Dictionary has about 470,000. The Deutsches Wörterbuch (DWB), the largest lexicon of the German language, has around 330,000 headwords. These values are cited by the dictionary makers and may not use exactly the same definition of a headword. In addition, headwords may not accurately reflect a dictionary's physical size. The OED and the DWB, for instance, include exhaustive historical reviews and exact citations from source documents not usually found in standard dictionaries.
The term 'lemma' comes from the practice in Greco-Roman antiquity of using the word to refer to the headwords of marginal glosses in scholia; for this reason, the Ancient Greek plural form is sometimes used, namely lemmata (Greek λῆμμα, pl. λήμματα).