[petekp]

Google Dictionary is one I use frequently, every day. Just double-click a word to see its definition.

https://chrome.google.com/webstore/detail/google-dictionary-...

[donut2d]

On Mac you can use a 3-finger tap on a world or select "Look Up In Dictionary" from the context menu to look it up in the dictionary and Wikipedia.

[Betelgeuse90]

A bit off-topic but since I got a Mac and found out about this - Google Dictionary became obsolete. The Mac dictionary works offline as well so I'm able to look things up during commutes without WiFi.