I am a logophile, and regularly looking up the meanings of words I encounter on the web or in pdf documents. More recent OS X versions include a great feature that allows you to get a quick definition in almost any app (browser, editor, etc). Highlight a word and press ctrl-command-d to get the definition of the word. It is easy to configure to include Wikipedia definitions so you can also look up people and concepts.
To customize which dictionaries are used (including wikipedia entries) open spotlight (command-space), type dictionary and press return to open the dictionary app. Open the preferences menu and tick the checkbox for the sources you wish to use. Once you make pressing ctrl-command-d a habit you will find using it is very natural, and requires little thought to remember it.
Make sure you have wikipedia configured and then try it on these:
- Thomas Bayes
- Markov chain
The pop-up definition is interactive so click on a definition to see the full definition entry then use the back arrow to get back to the list of definitions. You can also click on the source title to open the dictionary app with that entry.