Real Time -or- ReTweet
Actually it is Retweet, and when used this way, RT means that the tweet you're looking at was forwarded to you by another user.
Online jargon, also known as text message shorthand, used primarily in texting, online chat, instant messaging, email, blogs, and newsgroup postings, these types of abbreviations are also referred to as chat acronyms.
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NetLingo Classification: Acronyms and Text Message