ETYMOLOGY
from Latin decennalis (of ten years) + -an
EXAMPLE
“… But the war was finished in the first year of the fourteenth Olympiad, in which the Corinthian Damon conquered in the stadium, and when, among the Athenians, the Medontidae still held the decennalian government, and the fourth year of the reign of Hippomenes was expired. …”
From: The Description of Greece
By: Pausanias
Translated by: Thomas Taylor
Lexicophilia 