The Global Language Monitor (GLM) is an Austin, Texas-based entity that documents, analyzes and tracks trends in language and publishes a list of the year’s most used English words, names, and phrases.

According to GLM’s algorithm, 2009’s most used word, both online and in print, is Twitter.

GLM’s top ten for 2009:

Twitter
Obama
H1N1
stimulus
vampire
2.0 (as a suffix attached to the next generation of everything. Ex. Web2.0)
deficit
Hadron
healthcare
transparency

A look at the Words of the Year for 2000-2008 recalls the prominent events and personalities of those years:

2000 chad
2001 GroundZero
2002 misunderestimate
2003 embedded
2004 incivility
2005 refugee
2006 sustainable
2007 hybrid
2008 change

Taking the decade as a whole, here are the top ten words with GLC comments:

1. Global Warming (2000) Rated highly from Day One of the decade
2. 9/11 (2001) Another inauspicious start to the decade
3. Obama- (2008 )The US President’s name as a ‘root’ word or ‘word stem’
4. Bailout (2008) The Bank Bailout was but Act One of the crisis
5. Evacuee/refugee (2005) After Katrina, refugees became evacuees
6. Derivative (2007) Financial instrument or analytical tool that engendered the Meltdown
7. Google (2007) Founders misspelled actual word ‘googol’
Surge (2007) The strategy that effectively ended the Iraq War
9. Chinglish (2005) The Chinese-English Hybrid language growing larger as Chinese influence expands [There are an estimated 300 to 500 million users and/or learners of English in the People's Republic of China.]
10. Tsunami (2004) Southeast Asian Tsunami took 250,000 lives

To see the top phrases and names for 2009 and the first decade of the 21st century, explore the Global Language Monitor site.