Attrition is when employees decide to leave or retire, and are not replaced by their company.
Regrettable (or negative) attrition is voluntary turnover where high-performing or high-potential employees are leaving the company. This can lead to skill and knowledge gaps, increased workloads, and decreased engagement within the remaining staff.
Non-regrettable (or positive) attrition can be a way for companies to downsize and reduce labor costs, without laying off employees (which is a more deliberate act to reduce the number of people working in an organisation). This type of attrition might happen for a number of reasons, such as a reduction in the number of paying customers, generating smaller revenue, and a lack of new customers.