A classifier is a word or affix that expresses the classification of a noun.
In Spanish, the affixes -a and -o classify nouns according to feminine or masculine gender, respectively. Here are some examples of nouns they classify:
Mish, Frederick (editor). 1991.Webster's ninth new collegiate dictionary. Springfield, MA: Merriam-Webster. 1,564 pages. 0877795088; indexed 0877795096; deluxe 087779510X.
Crystal, David. 1980.A first dictionary of linguistics and phonetics. Boulder, CO: Westview.