Usually APIs require keys to limit the usage and offer different pricing and also to prevent denial of service attacks. However, I did find one that doesn't require any key. It uses dict.org, there is a C# client app consuming this api on this web page: http://www.aonaware.com/dictservicesample.htm.
It seemed a bit dated, so I tried it out and it still works. You can download the zip on that page and try the command line tool for yourself, just pass the arguments "define wheel" to the exe and it will output the definition of wheel. It supports a lot of different sources, you can limit your search to only one dictionary if you like. There is a help on all of the command line options of course.
You have access to the C# source code, it shouldn't be too complicated if you know C# or Java. Don't get confused with all the command line parsing code. If you are doing your app in a different language, you will only care about the calls made to the API, just read the code or use the app while capturing the HTTP traffic with Fiddler, it's a web proxy and it's fairly accessible. Hope it helps.
Reading is one form of escape. Running for your life is another. ― Lemony Snicket