A way to say hello and good bye at the same time, used when you see someone for the first time that day but have to leave immediately.

When you see someone for a short period of time

A person you would say "hi" and "bye" to casually, but little more. Generally someone desirable as an acquaintance and not anything more.

From "This American Life"

A brief meeting in a public place. Usually happens when running into someone you: a) barely know, b) don't like, or c) don't want to introduce to the people you're with.

A brief meeting in a public place (usually happens when running into someone you: a) barely know, b) don't like, or c) don't want to introduce to the people you're with).

People who say hi when you come to school/work and occasionally have sexual interaction with you

a ceremony in which an incoming executive is welcomed and farewell is bid to an outgoing executive.