a light metering metering mode, common in modern SLRs and present in some compact cameras.
In matrix metering, the output of a number of light sensors is weighted according to certain euristics.
Some systems have a very large number of sensors (the Nikon F5 has over one thousand).
Sensor output is compared against a database of patterns and used to pick an exposure.
Some cameras are smart enough to recognize certain very bitchy light situation, like photos where the background is sand or snow (this would result, if not properly exposed, in an underexposed picture, as the meter attempt to render the prevalent sand or white as 18% grey).
Not only it sounds like black magic: it is. Camera makers like to impress the buyer by saying that "... the scene is compared to thousands of pre-analyzed and metered pictures taken by our great photographer ... you too, the knuckle-dragging amateur photographer will be able to benefit of the experience of Galen Rowell ..."
Which is partly bunk. I use it only when I am in hurry, or when I am using a portable flash - especially with a Nikkor D lens.
Contrast with spot metering, center weighted metering. See also light metering for some general concepts.