decennial (adj.)

"existing or continuing for ten years; occurring every ten years," 1650s, with -al (1) + Latin decennium, from decennis "of 10 years," from decem "ten" (from PIE root *dekm- "ten") + annus "year" (see annual (adj.)). For vowel change, see biennial. As a noun, "a tenth anniversary," by 1884.