What is this socket/port/connector on my laptop? [duplicate]

Solution 1:

That is a Kensington lock used for anti-theft purposes. It is attached by means of a cable from something stationary, such as a table, to the device. The cable will then lock at one end from the port, preventing theft. This is much like a bike and it being locked to a railing or something being anchored to prevent removal. An example of a Kensington lock and cable on a PowerBook G4 (from Wikipedia):

Without the lock installed:

With the lock installed:

Here, the lock is attached to the corresponding port on the G4, and to the table, preventing theft of the device. Another example of the lock being used:

There are many kinds of these locks, by the Kensington company. See their page for details.