The definition that is widely accepted in academia is:
"Marketing is the process of planning and executing the conception, pricing, promotion, and distribution of ideas, goods, and services to create exchanges that satisfy individual and organizational objectives."
by American Marketing Association
Almost all people who took a course titled Introduction to Marketing are forced to memorize this definition as well as the 4Ps of marketing: Product, Price, Place and Promotion.
Product is the tangible (a can of soda) or intangible (car wash) item or service that you are trying to sell.
Price is the monetary value that the customers are asked to pay in exchange of the product
Place is the distribution efforts that you are making in order to sell the product.
Promotion is the efforts (other than distributing) that enables the customers become aware of your product.