in a phrase the headword is the word that is modified by everything else in the phrase. For example: in the noun phrase 'University lecturers with more than three years' experience' the headword is the noun 'lecturers'; in the prepositional phrase 'in this part of the body' the headword is the preposition 'in'.
A headword (or head word) is the word under which a set of related dictionary definitions will be listed. This is the word that will be used to locate the entry, and which dictates its alphabetical position within the dictionary. Depending on the size and nature of the dictionary, these definitions may include alternative meanings of the word, its etymology and pronunciation, compound words or phrases that contain the headword and any other related information deemed useful by the authors.