Halliday (2004: 3):
But teachers of English have customarily distinguished between content words, like snow and mountain, and function words, like it and on and of and the; and it is the notion of a content word that corresponds to our lexical item.
Halliday (1985: 61):
Grammatical items are those that function in closed systems in the language: in English, determiners, pronouns, most prepositions, conjunctions, some classes of adverb, and finite verbs. (Determiners include the articles.)