Re has several meanings:

• a legal term meaning "In reference to:" or "In regard to:". It is this meaning that is used by the email clients that prepend "Re: " to the original message's subject field when composing a reply.
• an alternate spelling of ancient Egyptian god Ra
• second pitch of solfege scale
• function returning real part of complex number
• short for reinsurer, as in Swiss Re or Fortress Re
• short for Rupee, a currency
• the symbol for the element Rhenium
• the Latin ablative of "thing;" check Wiktionary
• an English language prefix; check Wiktionary
• rematch in Internet slang
• an album by the Mexican band Café Tacuba

The abbreviation RE can mean:

• Religious Education
• Resident Evil
• Real Estate
• regular expression
• Recursively Enumerable
• Reverse engineering is the process of taking something (a device, an electrical component, a software program, etc.) apart and analyzing its workings in detail.
• Royal Engineers, a corps of the British Army
• the IATA code for Aer Arann
• rehi ("Hello Again"). A greeting originating, and occurring mostly, in Internet Relay Chat