There are a lot of ways to define a startup. Paul Graham defines a startup as growth.
This is how I would define it.
A startup is an interesting & brutal game. The reason why it is both interesting & brutal is that the rules of the game keep on changing. Every 2 or 3 years, almost the entire rule set will be replaced. It is even crazy to imagine playing cricket for the first 1 hour, soccer for the next 2 hours and basketball for the remaining 1 hour.
With the above definition, one can assume that playing by the same rules is a sure way to lose the game. Since no one knows when the rules of the game change, one can understand the rules only by observing the results of each round.
#meta – The above definition of a startup also changes every few years.