A benchmark is a test that measures the performance of a system or subsystem on a well-defined task or set of tasks. A benchmark for a graphics card, for instance, might test how many frames the card can draw per second; a benchmark for a Central Processing Unit (CPU) might test how many computations the CPU can perform per second.
Software benchmarks typically test how many transactions an application can process in a specified time frame. The Transaction Processing Council's TPC-C benchmark, for instance, is a common test used to calculate the number of orders processed by a database in a minute.