The United States Postal Service issued definitive stamps in sets with clearly defined themes and common design elements throughout most of the twentieth century. During the post-1940 or 'Modern' period, the U.S. Postal Service issued both traditional definitive sets as well as definitive stamps without a common theme, design, or name.
The major definitive series include the Liberty series, the Prominent Americans series, the Americana series, the Great Americans series, Transportation coils, Flora & Fauna series, and the Distinguished Americans series.
The non-traditional definitives—that is, those without a common theme, design, or name—can be broadly categorized as American Culture and Scenes, expedited mail, flags, wildlife, mini-scapes, and rate change issues. These stamps can be found in the catagories of Limited and Regulare Definitive Issues within Arago."
During the Modern period, definitives were issued in a variety of formats including sheets, coils, and booklets with self-adhesive or water-activated gum. They were printed by both private contractors and the Bureau of Engraving and Printing (BEP). A variety of perforation sizes was used and the period ushered in 'die-cut' separations on self-adhesive stamps.
The Postal Service announced in 1995 that future definitive issues for nonprofit bulk mail would be comprised of stamps issued in non-denominated form, their assigned values not printed on their faces.
Juell, Rodney A., and Steven J. Rod, eds. Encyclopedia of United States Stamps and Stamp Collecting. Minneapolis: Kirk House Publishers, 2006.