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What is Enterprise Architecture?

One definition of EA is the following: "Enterprise architecture (EA) is the process of translating  business vision and strategy into effective enterprise change  by creating, communicating, and improving the key principles  and models that describe the enterprise’s future state and  enable its evolution."

This, however, might not be really helping you further. Have a look at this e-book to learn what EA is in a more visual and understandable manner.