A common metric that is used to measure the amount of people that leave an organisation, typically expressed as a percentage.
If you want to get a little technical you can have internal attrition – when staff leave to work in other departments/sections within the same organisation (which is not necessarily a bad thing!) and external attrition, when staff leave the organisation altogether (normally a pretty bad sign).
And because you are here and wanting to learn, there is also positive attrition (losing people that can be replaced with someone better skilled, qualified or motivated) and negative attrition (losing talent and skills that you would have preferred to hang onto!).
In Australia, the widely used figure is 40% turnover per annum for the call centre industry. In offshore call centres there has been examples of figures as high as 140%.