[nashashmi]

I prefer Chrome's extension called Google dictionary. Every time I highlight a word, it pulls up a definition. Anywhere on the web.

[err4nt]

OS X and iOS users can do this in Chrome and Safari, I think the shortcut it selecting a word and pressing Command-Control-D or something :)

[neic]

Select and Command-Control-D is right. You can also do a three finger tab. See https://en.wikipedia.org/wiki/Dictionary_(software)#Quick_ac...