happening every eleven years
• (a.) Occurring or observed every eleventh year; belonging to, or continuing, a period of eleven years; undecennary; as, an undecennial festival.
undecennial Related 'metric' families: yotta; zetta; exa; peta; tera; giga; mega; kilo; hecto; deka; centi; milli; micro; nano; pico; femto; atto; zepto; yocto.