Volumetric efficiency
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In internal combustion engine design, volumetric efficiency refers to the efficiency with which the engine can move the charge into and out of the cylinders. More correctly, volumetric efficiency is a ratio (or percentage) of what volume of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions. Therefore, those engines that can create higher induction manifold pressures - above ambient - will have efficiencies greater than 100%. Volumetric efficiencies can be improved in a number of ways, but most notably the size of the valve openings compared to the volume of the cylinder. Engines with higher volumetric efficiency will generally be able to run at higher RPM, and thus power, settings as they will lose less power to moving air in and out of the engine.
There are several "standard" ways to improve volumetric efficiency. The most common is to use a larger number of valves, see multi-valve, which cover a greater area of the cylinder head. Automobile engines typically have 4 valves per cylinder today for this reason. Many "high performance" cars in the 1970s used carefully arranged air intakes and "tuned" exhaust systems to "push" air into and out of the cylinders due to airflow over the engine. A more modern technique, variable valve timing, attempts to address changes in volumetric efficiency with changes in RPM of the engine -- at higher RPM the engine needs more time of a single cycle to move the charge out of the engine.
More "radical" solutions include the sleeve valve design, in which the valves are replaced outright with a rotating sleeve around the piston, or alternately a rotating sleeve under the cylinder head. In this system the ports can be as large as you need, up to that of the entire cyclinder wall. However there is a practical upper limit due to the strength of the sleeve, at larger sizes the pressure inside the cylinder can "pop" the sleeve if the port is too large.
Also see: supercharger, turbo charger, engine tuning.