Alpus

Alpus is a Java-based, comprehensive piece of software that enables you to quickly look up definitions of words within a well-organized main window.

Works as an online or offline dictionary and as a Wikipedia browser

The first thing you should know about this app is the fact that it is designed to make it possible for yo to access multiple dictionaries at once, without necessarily requiring Internet access.
This said, the program is fully compatible with StarDict, DSL, XDXF files offline dictionaries, as well as with ZIM and Wikipedia offline files. This is a good time to point out that you can easily download new dictionaries from the Internet or, if there is no Internet connection, import them directly from your computer's hard drive.

It may not look apart but the interface is quite intuitive

Getting started with this app is by no means a difficult task since, aside from making sure that Java is installed on your computer's system, you simply need to unzip the package and run it via its executable file.
Upon launching the app, you are met by a well-organized main window with a typical Java interface. Most of the app's main features can be easily accessed from the toolbar from the bottom toolbar, more specifically, from the app's pop-up main menu.

Packs a useful set of tools

The app offers you multiple options to quickly search for words, as well as other, very useful, features. For example, you can perform local or online word searches, view an accurate history of your searches, as well as bookmark your favorite or most-used words.
Other self-explanatory capabilities like Clipboard Scanner, SU Viewer, a night mode, Vocabulary Exerciser are also part of the reason why this app is so functional and useful. What is more, the utility enables you to export your dictionaries, create .zip archives, extract files, and also to convert .zip archives.

Full-featured and cross-platform learning tool

All in all, Alpus is a dependable utility that provides an intuitive user interface, extensive word search capabilities, both online and offline, and extensive set of useful tools that deem it one of the most versatile learning utilities of its kind.
Last but not least, since it's Java-based, you can take advantage of everything it has to offer on computers with Windows or Linux and on Macs.