A surprise party is a party that is not made known beforehand to the person in whose honor it is being held.
Birthday surprise parties are the most common kind of surprise party. At most such parties, the guests will arrive an hour or so before the honored person arrives. Often, a friend in on the surprise will lead the honored person to the location of the party without letting on anything.
The guests might even conceal themselves from view, and when the honored person enters the room, they leap from hiding and all shout, "Surprise!" For some surprise birthday...