The rate at which visitors to a website complete the predefined goal. It is calculated by dividing the number of goal achievements by the total number of visitors. For example, if 100 people visit a website and 10 of them complete the conversion goal (like filling out a contact form) then the conversion rate is 10%.
In social media terminology, conversion rate is the percentage of users who see your post or ad who then take a specified action. That action is called a conversion, and it could mean purchasing an item, signing up for a newsletter, downloading an ebook, or a variety of other acts.