The Oxford English Dictionary defines "tip" in this usage as the bestowing of "a small present of money upon an inferior, especially upon a servant or employee of another, nominally in return for services rendered or in order to obtain an extra service."
Learn about the controversial history of tipping.
Tipping practices vary greatly around the world. In some places, it's an extra amount for good service, and in others, it's expected that you will leave some sort of a tip, even if the service was simply adequate. In another variation, some restaurants will apply a service charge to your bill in lieu of a tip. And in some countries, like Japan, a tip is insulting.
Learn about country-specific tipping guidelines here.
Handling sticky situations: If the owner of a restaurant waits on you, do you need to tip?
