"occurring every 11 years," 1858, in reference to solar activity cycle, from Latin undecim "eleven" + ending from biennial, etc.
Entries linking to undecennial
biennial adj.
1620s, "lasting for two years;" 1750, "occurring every two years," from Latin biennium "two-year period," from bi- "two" (see bi-) + annus "year" (see annual (adj.)). The vowel change is "due to the Latin phonetic law according to which the unaccented and closed radical syllable of the second element of compounds, original -ă- becomes -ĕ-" [Klein]. The noun meaning "a biennial plant" (which requires two seasons of growth to produce flowers and fruit and dies the next) is attested by 1770. Related: Biennially.