A very good one, with the exception of a few disadvantages like limited vocabulary
Edit: Doesn't work anymore. Such a shame. :(
Edit2: Thank you so much for the user who recommended 'definer' by lumetrium! Even though its not as simple as this one was, it does the job.

This used to work great, but now ever word is searched for and not found. Rendering this completely useless.

It's terrible. It doesn't provide results for nearly every single lookup. I'd say it's a bust!

Sorry

No definition found.

ON EVERY WORD!

Last update in Dec 2020. Please report this abandoned extension instead of installing.

It used to work well, but it won't define any words anymore. All I get is "No definition found"

"No definition found" for all words that I tried, including "music" and "the".

Doesn't work anymore, says "No definition found" for all words

Doesn't work when I double click on a word

Doesn't work.

useless! About 1 in 50 words get a definition.

Doesn't even work.

Majority of the definitions I selected on a webpage comes back with "no definition found", and I am talking about the simplest words like "asinine" and "relationship".

There used to be Dictionary Search add-on with many configurable URLs. It is now gone, and instead we have this piece of crap with the single URL that opens a new window. Utter joke.

No definition found almost every word.

Majority of the definitions I selected on a webpage comes back with "no definition found", and I am talking about the simplest words like "asinine" and "relationship".

Continuously get the "no definition found", tried turning off all my other extensions but that wasn't it, seems to be a problem wit the extension itself? Reinstalling did not work.

My language is set to (US) English. I click a few words on a French page.
'propres' brings up a French definition, not an English translation 'propre: Qui appartient d'une manière exclusive ou particulière à une personne, une chose.'
Double-clicking 'permet' displays 'permit: Give authorization or consent to (someone) to do something.'
'invitent' gives 'No definition found'
'nombre' results in a Spanish definition, 'nombre:
Palabra o conjunto de palabras con las que se designan y se distinguen los seres vivos y los objetos físicos o abstractos.'