ManyThings.org - ManyThings is a multiplatform and user friendly application for students of ESL or EFL to learn the meanings and definitions of different terms and phrases.
Reverse Dictionary - Describe a concept and get back a list of words
SlangSite.com - SlangSite.
Writefull Thesaurus - A Google Docs add-on that uses AI to give you synonyms
InternetSlang.com - InternetSlang.
Meaning Monkey - Find the definition and meaning of anything.