Quik Dictionary od Wicked Writer

How to use it

Double-click any word on a web page.
A small popup appears next to the word with its definition and part of speech.
Press Escape or click anywhere else to close it.

Features

Instant definitions for English words, including slang, technical terms, and less common vocabulary
Shows multiple parts of speech with concise definitions
Popup is styled separately from the page, so it looks the same on every site
Automatically follows your system's light or dark theme
Lightweight, with no account or setup beyond a one-time permission

First-time setup
Click the Quik Dictionary toolbar button once and accept the permission prompt. This allows the extension to look up definitions from Wiktionary.

Data source
Definitions come from Wiktionary, a free, community-edited dictionary run by the Wikimedia Foundation, and are available under the Creative Commons Attribution-ShareAlike license.