A gross simplification of an "art installation" is an artwork that "interacts" with the space/environment surrounding it or those who are viewing/experiencing it.
The "installation art" article from Answers.com offers the following definition:
installation art
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Similarly, a "monolith" is huge block of stone carved into the shape of a column or a sculpture, you can also read more about these in Answer.com's "monolith" article. Recognisable examples of these are the Moai on Rapa Nui - in other words the big stone heads on Easter Island!
So what do I think I'm going to for this project - build a full scale megalith and film it?