How tweet it is: Tweet added to Oxford English Dictionary
The word "tweet" itself dates back as far as 1851 when it was first used as a description of a bird call. The definition of tweet now includes "to make a posting on the social networking service Twitter. Also: to use Twitter regularly or habitually." Ironically, the Oxford English Dictionary included the word "retweet" back in 2011, two years before the acceptance of "tweet".
So kids, now that "tweet" is a noun or a verb, don't be afraid of using it in your reports. And if your teacher takes off points for using the word incorrectly, just pull out a new copy of the Oxford English Dictionary and let her catch up with the times.
source: OED via AllThingsD