The word "selen-" is derived from the Greek word "selene," which means moon. Synonymous words for "selen-" include "lunar," "lunatic," and "selachian." "Lunar" refers to anything related to the moon, while "lunatic" is used to describe someone who is insane or has unpredictable behavior, possibly due to changes in the moon's phases. "Selachian" is a term used to describe any species of fish in the shark family, which were once believed to be under the influence of the moon. Other synonyms for "selen-" may include the names of different lunar phases, such as "full moon" or "new moon".