What is Matrix Management?

Legal Definition
Strictly speaking, matrix management is the practice of managing individuals with more than one reporting line (in a matrix organization structure), but it is also commonly used to describe managing cross functional, cross business group and other forms of working that cross the traditional vertical business units – often silos - of function and geography.
-- Wikipedia
Legal Definition
Employees with dual responsibilities and dual bosses have multiple command-and-control structures. These employees have one boss, as project manager, for example, for day-to-day operations. They have another boss, a departmental head, for example, for functional responsibilities. Large projects or product development processes are most suited to this approach, dealing with situations with fluctuating workloads. Also refer to matrix organization.