1) Content word 2) Head word 3) Open-class word 
A headword, head word, lemma, or sometimes catchword is the word under which a set of related dictionary or encyclopaedia entries appear. 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 e...word forming the heading of an entry in a dictionary.
[n] - a word that is qualified by a modifier 2. [n] - a word placed at the beginning of a line or paragraph (as in a dictionary entry)
noun a word placed at the beginning of a line or paragraph (as in a dictionary entry)
one of the list of words in a dictionary, that is followed by an explanation of what it means