Let us start with Wikipedia, even though it is not the most authoritative source on the topic of Enterprise architecture (EA)
"a well-defined practice for conducting enterprise analysis, design, planning, and implementation, using a holistic approach at all times, for the successful development and execution of strategy. Enterprise architecture applies architecture principles and practices to guide organizations through the business, information, process, and technology changes necessary to execute their strategies. These practices utilize the various aspects of an enterprise to identify, motivate, and achieve these changes."
On the second part of your question, why do we do it? The short answer: Successful Enterprise Architectures bring together the Business, Information, Data, Application and Technology (BiDAT) dimensions of an enterprise. When practiced successfully, EA can enable strategy realization by bringing in new capabilities, and enhance existing capabilities.