In the post “Merriam-Webster and OED add new words: Lorem ipsum, TL;DR, and more” from the Columbia Journalism Review, Merrill Perlman (@meperl) succinctly differentiates between the Merriam-Webster Dictionary and the Oxford English Dictionary (OED):
Merriam-Webster is “synchronic,” meaning it concentrates on current, active vocabulary. The OED is “diachronic,” written from a historical perspective.
It’s this diachronic element—which gives the historical usage of a word—which I find so useful, and which makes my yearly OED online subscription one of my most valuable writing tools.
Why? Because every time I dive into the OED with a specific word, because it focuses on the origins of the word by showing a range of examples of its prior usage, my concept of that word invariably broadens.
Effectively, the OED is an intellectually expansive and creative tool for a writer—or at least this writer!
(One friendly word of caution though. It can be so much fun exploring the OED it’s easy to get lost in the joy of linguistic exploration. Getting in and out of the OED in less than 5 minutes (as opposed to one hour) takes real discipline.)