the GNU version of the Collaborative International Dictionary of English
adj. Comprising four years.
adj. Occurring once in four years, or at the end of every four years.
The Century Dictionary and Cyclopedia
Comprising four years: as, a quadrennial period.
Occurring once in four years: as, quadrennial elections.
n. A fourth anniversary, or its celebration.
Word Usage
"The administration hasn't finished a mandatory comprehensive study of U.S. nuclear-arms policy or a broader study of defense, known as the quadrennial review."