Language is constantly evolving, and the past few decades have been very good ones for the English language. Words such as "gigabyte" would have been near meaningless 30 years ago, and now your average 12-year-old knows how many "gigs" a laptop hard drive is.
Unsurprisingly, more words are constantly being added, and the Oxford English Dictionary has introduced a new definition for "tweet." The Dictionary has therefore broken its own rules, that a word must be in use for a decade or more before inclusion. Twitter is seven years old at the moment, so they're three years out. This is perhaps not too surprising, for they're pushing to revise the Oxford English Dictionary.
"Follow" has also been revised to include tracking someone on a social media website, while the words "flash mob" and "live blog" are also new entries. Flash mobs are groups of people who meet at a certain point, often in droves, having arranged it over the Internet.
The speed language evolves at makes it difficult to remember all these changes, Google's handy "define:
Source: PC World