Ave Legio literally means "Hail Legion" in Latin and has been the Legion's de facto motto since 2006. The use of the term dates to the Cyber Nations forums where hailing is common throughout all alliance announcements. To distinguish itself from the standard hail, Legion members adopted "Ave" instead of the typical "o/" hail.
Person A: The Legion has grown by 50 nations this quarter!
Person B: Ave legio!