What exactly is a startup?

Everyone has their own definition of just what a startup is, and there's no definition any two entrepreneurs or investors agree on. Most say a startup is determined by its age, growth, revenue, profitability or stability.

According to Wikipedia,
A startup company (startup or start-up ) is an
entrepreneurial venture typically describing newly emerged, fast-growing business . Definition of the startup usually refers to a company , a
partnership or an organization designed to rapidly develop scalable business model.

Here are  definition startup from some of the most notable entrepreneur

Stanford professor Steve Blank, for example, describes it as "an organization formed to search for a repeatable and scalable business model.

Eric Ries' definition of startups in his book "The Lean Startup":
A startup is a human institution designed to deliver a new product or service under conditions of extreme uncertainty.

Paul Graham says that "A startup is a company designed to grow fast. Being newly founded does not in itself make a company a startup. Nor is it necessary for a startup to work on technology, or take venture funding , or have some sort of " exit ". The only essential thing is growth.

People argue relentlessly about what a startup is, but personally I prefer One popular definition by Steve Blank and Bob Dorf defines a startup as an "organization formed to search for a repeatable and scalable business model

Blank and Dorf add that startups are not smaller versions of larger companies: a startup is a temporary organization designed to search for a product/market fit and a business model,

1)Temporary Organization

Startup's goal is to cease being a startup, to graduate up to being a large business or to fail and move on to another opportunity.

2)Search
"search" because it is a key distinction from small and large businesses that operate in mature markets. These businesses execute known business models and products  while a startup searches for an unknown  business model
You use very different techniques to search
effectively and efficiently for something unknown than you do to execute something that is already known.

3)Business model
Successful operations of business identifying
value prepositions(product) , revenue, customers ,and channels   which are scalable and repeatable
A business model describes how your company creates, delivers and captures value.
Or in English: A business model describes how your company makes money.