WordWeb v5.52 Pro Dictionary was made portable using Thinstall, which means it can be run from a USB drive without first installing and while maintaining your settings (a WWSettings folder is created on first use).
This dictionary stands out from the rest because it includes a thesaurus on steroids, anagrams, allows you to add your own words or pre-compiled word lists ... Since this is only a collegiate dictionary I also recommend obtaining RH Webster's Unabridged Dictionary WG v4.7 (RH = Random House & WG = Word Genius), which I also made portable. Enjoy.
Note: This app is distributed inside a self-extracting 7zipped exe file in order to save space (18.2 MiB down to 8.75). I normally compress the app itself, however, this resulted in MUCH slower full-text searching.