What is the HEART metric?

Work methodology developed by Google to measure user experience in digital products, which includes five categories with indicators (Happiness, Engagement, Adoption, Retention, Task success: HEART). Happiness (happiness) measures the user's attitude towards the system, engagement evaluates the level of involvement of the user with the product, adoption (adoption) is the number of new users of a product or a product characteristic, retention (retention) it is the rate at which users return to using a product and task success (tasks) includes traditional metrics of behavior in user experience.