The act of looking into someone's eyes who, in return, is looking into your eyes. Eye contact has many uses, often invovling communication. Here are some examples:

Interviews:
Good eye contact during a job interview is right up there with a firm handshake and an unfaltering smile. If you screw up in any way, the job is lost. If you figure it's pretty much lost anyway, feel free to stare at your thighs the whole time.

Presentations:
Good eye contact is also a plus for presentations if both you and the audience care about its content. In the case of many speeches, only the speaker cares about the content, and in the case of boring technical presentations, usually both the presenter(s) and the audience just want to zone out until its over.

Communication:
Sometimes you can communicate things to people simply by using your eyes. But first you must acquire eye contact with them. This can be frustrating as hell if you're trying to communicate with someone who never looks your way.

Supernatural:
Many people say they can feel eye contact.

Discipline:
See: Evil eye

Games:
See: staring contest

Flirting:
See: Templeton's how to flirt node
and the eye contact game

Lust:
See: 70 minutes of bliss