In the context of marketing, conversion refers to the act of turning a prospect or visitor into a paying customer, subscriber, or lead. It's the ultimate goal of most marketing campaigns and the measure of success for marketing efforts. The more conversions that a campaign generates, the more successful that campaign is considered to be.
A conversion occurs when a user takes a desired action, such as filling out a form, making a purchase, signing up for a newsletter, or requesting a demo. This action is typically defined by the marketer and aligned with the business goals.