Freelang Dictionary is a free program for Windows. It is very easy to install and use: choose a language from the list after installation. You can then download it and add it to the program. 
After installing the Freelang main application, you can download and install as many languages as you like. The program lets you browse lists, search for a word, add or modify an entry, and learn new words from a personal learning list.
All keyword lists are bilingual (from and to English). The app has other combinations (such as from and to French or from and to Spanish). The program runs on all versions of Windows. It's free, but the lists are copyrighted by their authors.
If you can't download the dictionary or want to rate it first, you can use it online version (works for all platforms: Mac, Linux ...).
Currently, 270 dictionaries are available. These include Greek.
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- Greek-English dictionary at the online bab.la language portal
- G. Babiniotis - Small Dictionary of Modern Greek Language for iPhone
