The means by which a camera automatically determines exposure settings such as shutter speed, aperture, and ISO (sensitivity).
Metering is a feature of modern cameras that measures the incoming light from the lens to automatically determine the correct exposure settings for shutter speed, aperture, and sensor sensitivity (ISO). Most DSLR cameras provide configurable metering options, including evaluative or some form of advanced metering, center-weighted, partial and spot. The various metering modes use a smaller and smaller area near the center of the frame to determine exposure. Some cameras allow spot metering to be based off of a selected auto-focus point, while others only make use of the center point.
Automatic camera metering can be very effective in many scenarios, however it is not fail-proof, and may result in an improper exposure. Some examples of this would be when photographing snow or a scene full of high-key objects. Compensation for improper metering is often possible in partial-manual or program modes of many modern cameras.