Probably the most famous American dictionary is the controversial Webster’s Third New International Dictionary, which made headlines when it was first published for taking a hard-line descriptivist stance, particularly for its treatment of the word ain’t. It remains the most important unabridged dictionary of American English, although it hasn’t received a major revision since it was first published in 1961.
The Merriam-Webster Collegiate Dictionary, which is shorter and can be held in one hand, remains Merriam-Webster’s flagship dictionary, receiving frequent updates and revisions. It is also the best-selling dictionary of American English. The online version is accessible at Merriam-Webster.com.
The other significant dictionary of American English is the American Heritage Dictionary, which was published by the owner of the history magazine American Heritage, who was appalled by the permissiveness of Webster’s Third. The American Heritage Dictionary is notable for pioneering the use of corpus linguistics in dictionary compilation, as well as for its 200-member “usage panel” who the dictionary consults when writing usage notes, reporting what percentage of the panel approved or disapproved of different contested usages.
There are othe American dictionaries which are well-regarded but not as well-known, such as the Oxford American, Random House, and Webster’s New World. Americans generally don’t distinguish among dictionaries and typically refer to all dictionaries as “the dictionary”, as in “I’ll look that up in the dictionary”.