Reinforce the things you look up by getting periodic reminders of them over time.
This extension will remind you of words or concepts which you've researched in the past at designated intervals in the future, in an attempt to exploit the spacing effect to reinforce the knowledge over time.
You can search from the pop-up or by selecting text and right-clicking on it.
It's currently designed around the idea of looking up words but it'll work for phrases and such, too. See the website for release notes and for a peak at what's coming down the pike.