I've seen this on the wall-facing side of many (North American) AC power connectors.

What is it called? What does it mean?

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It's a kind of datecode. The mold for the plug actually has a small part that can be rotated with a screwdriver, to point the arrow at different numbers around the perimeter of the circle. They do this to either indicate a new month of manufacture, or perhaps a revision number.

Note the 1 and the 3 on either side of the arrow. In this case, it's a datecode indicating the third month of 2013. Once a month, they rotate the arrow, and once a year, they replace the rotating part altogether with one for the next year.

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    \$\begingroup\$ In addition. This kind of mark can be found on all sorts of injection molded parts, not only plugs. \$\endgroup\$ – Nick Alexeev May 3 '14 at 1:03

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