This is the 507th most frequent Hungarian word.
említett
"Mentioned"
Here, 'említett' is used as an attributive adjective to describe the noun 'könyv' (book), meaning 'mentioned'.
Az említett könyv a polcon van.
The mentioned book is on the shelf.
Here, 'említett' is again an attributive adjective, providing additional information about 'things', specifically that they were previously mentioned in the context.
Az említett dolgok mind fontosak.
The mentioned things are all important.
In this case, 'említettük' is the past tense, first person plural form of the verb 'említ' (to mention). It describes an action performed.