Instadict

Update: Google restricted the clipboard access to background apps since Android 10 so Instadict won't work on Android 10+

Source - https://www.xda-developers.com/android-q-blocks-background-clipboard-access/

Instadict lets you view a word's meaning anywhere!
Just copy the word and shake your device horizontally
to get it's meaning. No need to interrupt what you're doing.

It also supports full offline mode! Just download the appropriate dictionary from the link below and you're all set! It is suggested you download them for an overall better experience.

English : https://www.google.com/url?q=http://play.google.com/store/apps/details?id=livio.pack.lang.en_US
French : https://www.google.com/url?q=http://play.google.com/store/apps/details?id=livio.pack.lang.fr_FR
Italian : https://www.google.com/url?q=http://play.google.com/store/apps/details?id=livio.pack.lang.it_IT
Spanish: https://www.google.com/url?q=http://play.google.com/store/apps/details?id=livio.pack.lang.es_ES

Licenses :
jsoup : http://jsoup.org/license
Wiktionary : https://creativecommons.org/licenses/by/3.0/us/