Define "Adoption" within the context of the HEART framework.

In the context of the HEART framework, "Adoption" specifically refers to the number of new users who start using a service. This metric is crucial for determining how effectively a product is gaining traction in the market. It reflects the initial acceptance and integration of the service into users' routines, highlighting user engagement and the success of marketing efforts. A higher number of adopters indicates that the service is meeting user needs and has appeal, which is vital for long-term growth.

The other options focus on different aspects that do not align with the concept of adoption. For instance, measuring the volume of data stored pertains to storage capacity and data management rather than user engagement. Similarly, frequency of server usage is more related to user activity after adoption, while the age of server hardware has to do with infrastructure considerations rather than user behavior. Thus, understanding "Adoption" as the introduction of new users to a service is key to evaluating the initial impact and reach of a product in its early stages.