annual (n.) Look up annual at Dictionary.com
c. 1400, originally "service commemorating the anniversary of a person's death," from annual (adj.) or from Late Latin annualem (nominative annualis). By 1680s as "plant that grows again or blooms every year," also as "annual literary publication."
annual (adj.) Look up annual at Dictionary.com
late 14c., "appointed by the year;" c. 1400, ""occurring or done once a year," from Old French annuel "yearly" (12c.) or directly from Medieval Latin annualis "yearly," corresponding to Latin annalis as adjective form of annus "year."

This is reconstructed to be from Proto-Italic *atno- "year" (compare Oscan akno- "year, holiday, time of offering"), from PIE *at-no- "which goes," also "a year" (as "going around"), suffixed form of root *at- "to go" (source also of Sanskrit atati "goes, wanders," atamana- "to travel, wander," atya- "steed, runner"). The root also has Germanic derivatives meaning "a year," such as Gothic aþnam (dative plural) "year."