During 2004 blogs, or web logs, have become hugely popular and some have started to influence mainstream media.
Merriam-Webster said "blog" was the word that people have asked to be defined or explained most often over the last 12 months.
The word will now appear in the 2005 version of Merriam-Webster's printed dictionary.
"Now we're getting words that derive from it such as 'blogosphere' and so on," he said.
"But," he added, "it's a pretty recent thing and in the way that this happens these days it's got established very quickly."
According to US research firm Pew Internet & American Life a blog is created every 5.8 seconds.
Top 10 words of 2004 according to Merriam-Webster:
7. peloton : noun (1951) : the main body of riders in a bicycle race
Perhaps #10 there was looked up by folks wondering what ever came of democracy in America.