Billable hours are the time professional services firms charge to clients, the core unit of law, accounting and consulting economics. Accountants often call the same idea chargeable time.
The workplace connection is friction: every minute a fee earner spends hunting for a desk, a quiet room or a colleague is unbillable overhead. Firms that make the office effortless to use recover real chargeable time, which is why professional services firms are disproportionately heavy users of desk and room booking.