To find marketing definition is an easy task as there are many versions and just ask google you find hundreds. The issue is to find a GOOD definition. Drawing on one of the great marketing gurus Philip Kotler, we can also find different definitions as this author has published more than 50 books in his more than 50 years as Professor at the Kellogg School of Management.
To read different definitions I recommend the wikipedia where you can find the extended definition as Kotler says that «marketing is a social and managerial process by which individuals and groups meet their needs by creating and exchanging goods and services.»
The American Marketing Association AMA in 2007 released a new version that Marketing is «Marketing is the activity, set of institutions, and processes for creating, communicating, delivering, and exchanging offerings that have value for customers, clients, partners, and society at large. »
The last contribution of philip kotler marketing is the concept of 1.0, 2.0 and 3.0. We can find a video where he explains in a simple way, with the 1.0 marketing reach the mind of the consumer, marketing 2.0 to the heart of consumer marketing and finally the 3.0 add to the above, after the planet.
I would also add another definition of Beatriz Navarro Starbucks marketing manager who gave a lecture on marketing days and he left us his version of what is marketing. From the minute you find 8.30. You can see the video on CEU Media