Weblioポップアップ英和辞典 is an official extension by GRASグループ株式会社 that provides a popup dictionary feature and smooth translation navigation when selecting text on web pages. By hovering over English words on a webpage or double-clicking on them, users can view the meanings from Weblio英和辞典 or navigate to the translation results from Weblio英語翻訳.

The extension offers various features, including a mouseover dictionary that displays explanations for English words and idioms in a tooltip-like popup window. Users can also specify "search by selecting the English word" to easily view translations by double-clicking. The action to trigger the popup can be customized to Ctrl key and mouse hover combination. It supports plural forms of nouns and conjugated forms of verbs, eliminating the need to manually modify the word before searching on Weblio. Additionally, users can search for phrases and idioms when selecting the English word for searching.

Weblioポップアップ英和辞典 is a specialized mouseover dictionary extension focused on English word lookup. It does not respond to words containing Japanese, allowing for smooth copying of Japanese words on web pages. With the mouseover function, users can quickly search for translations and meanings of English words and phrases. It can also be used as a simple translation tool.