Wiktionary is the Wikimedia project's dictionary. Here is an example of a page, although the definitions are still quite technical:
https://en.wiktionary.org/wiki/data#English
Additionally, like simple.wikipedia, there is a Simple English Wiktionary page. Here is an example:
http://simple.wiktionary.org/wiki/data
The definition you are looking for:
Data is information, such as facts, numbers, photographs, etc.
The license is GNU Free Documentation License (text).
DOWNLOAD
All the Wikimedia projects use the same data dump scheme (backup index).
The Simple Wiktionary line looks like this:
2014-12-12 20:44:20 simplewiktionary: Dump complete
And the most recent link is:
https://dumps.wikimedia.org/simplewiktionary/20141212/
From this page, you are looking for the top files:
Articles, templates, media/file descriptions, and primary meta-pages, in multiple bz2 streams, 100 pages per stream
The index file (simplewiktionary-20141212-pages-articles-multistream-index.txt.bz2, 144 KB) contains a listing of all the pages. Here are some randomly selected records:
You can use these words to then find definitions from the pages. I extracted the XML dump and put on pastebin the data for the page "Data" - LINK. The part you are looking for can be parsed out from these lines:
{{noun3|data|datum|data}}
#{{uncountable}} '''Data''' is [[information]], such as [[fact]]s, [[number]]s, [[photograph]]s, etc.
#: ''A [[study]] using [[experiment]]al '''data''' from ten schools found no [[difference]] between [[method]]s.''
IMPORT
Here are some instructions on how to import the XML data dump - LINK.
Some possible tools, depending on your skills:
xml2sql (multiplatform C tool)
Pywikibot (Python, requires registering your app, but doesn't require reconstructing database)
mwdumper (Java)