An air dam fits under the front of the car, preventing most air from traveling under it. Sometimes it is called an air spoiler, but true spoilers are usually fitted to the rear. Air dams were part of the Datsun 1200 FIA homologation.
Classic old-school air dam "front spoiler"
Also see: Spoiler (rear spoiler and wings)
Modern air dam with brake ductwork
Materials
Most air dams are made out of wikipedia:fiberglas. Modern ones are made out of wikipedia:ABS plastic.
Donor Parts
The most popular OEM air dam to fit a Datsun 1200 is a Daihatsu Mira part.
Also see: Parts Reference