What is the Second ?

Source: en.wikipedia.org

" The second (symbol: s, also abbreviated: sec) is the base unit of time in the International System of Units (SI) (French: Système International d’unités), commonly understood and historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each. (...) "