Dictionary definitions never say enough. Every child knows that a party is more than just a gathering assembled to celebrate someone's birthday. But no brief definition can describe the complicated customs, rules, and regulations that typical communities prescribe for such ceremonies. When I was a child, a birthday party could be expected to include at least the elements of the following script:
ARRIVAL. Greeting. Be well dressed. GIFT. Give birthday present to host or guest of honor.
GAMES. Activities like blindfold competitions. DECOR. Balloons. Favors. Crepe-paper decorations. PARTY-MEAL. Hotdogs, candies, ice-cream, etc. CAKE. With candles to represent the host's age. CEREMONY. Host tries to extinguish candles with single breath (to make a wish). SONG. All guests sing birthday song and eat cake.
This is merely an outline, for every item leads to other conditions and requirements. The birthday present has to please the host, of course, but there are other strong constraints on it as well. It ought to be brand new, of good quality, and it should not be ostentatiously extravagant. It ought to be suitably party-wrapped — that is, covered with a certain kind of color-printed wrapping paper and tied with colored ribbon. There are also constraints on other items in the script. The birthday cake should be covered with a sweet sugar frosting. In my childhood, the ice cream usually consisted of three colored stripes of different flavors: vanilla, strawberry, and chocolate. Because I did not like the strawberry flavor, my personal party script included the extra steps of finding another child willing to make a trade.
To all their young participants, such parties unfold exactly as a party should, with all these queer complexities. We take our social customs for granted, as though they were natural phenomena. Few guests or hosts will ever wonder why their parties have those explicit forms or ask about their origins. As far as any child can tell, that's just how parties ought to go; they always did and always will. And so it is with almost everything we know.