What is the meaning of 'RT' in Twitter?

Retweet/Retweeting

When you come across an interesting tweet and you want to help it spread - you retweet it.

There's usually a ReTweet button next to each Tweet, so you can usually just click that.

The syntax of a ReTweet is the abbreviation "RT" or the word "Retweet" followed by the username of the person who tweeted it (e.g @User) and then the content of the actual tweet.

You may also modify the original tweet to make it more interesting for your followers, or because the addition of RT + username makes it longer than the permitted 140 characters. In order to make everything fit, you may need to omit words or use shorter synonyms or abbreviations.