Here's why:
* Noun: A noun is a word that names a person, place, thing, or idea. "Constraint" is a noun that refers to a limitation or restriction.
* Phrase: A phrase is a group of words that act as a single unit of meaning but do not have a subject and verb combination. "Design constraint" acts as a single unit that refers to a specific type of limitation.
Therefore, "design constraint" functions as a noun phrase, specifically a compound noun.