A noun or adjective that defines all single-word nouns (i.e. pronouns, proper, collective). This excludes multi-word nouns such as compound-nouns.
- Noun: Single-word nouns are called ______.
- Adjective: Single-word nouns are called ______-nouns.
I think the word you are seeking is "simple". If a noun is used instead of a noun phrase, that noun is a simple noun. A simple noun is not define by the number of words, but by the fact that it is not a noun phrase. However, in almost all cases, a simple noun is one word.