the most useful definition of “market” is 100% consumer-centric.

A market consists of all the consumers who can search for and compare products for a use case they already have in mind.

The most concrete test of pre-existing demand is using the Google Keyword Tool, which tells you how many people are searching on Google for a particular keyword.

What keyword do people search to get to your site?
Put those keywords into Google Keyword Tool
How many people are searching for this keyword?

If the answer to #3 is large (millions or more), then you have a large market.

For consumer internet, a great market is commonly defined by:

a large number of potential users
high growth in # of potential users
ease of user acquisition

even if there’s a ton of competition, if it’s easy to acquire consumers to your product, that’s great!

Leading with a great market helps you execute your product design in a simpler and cleaner way. The reason is that once you’ve picked a big market, you can take the time to figure out some user-centric attributes upon which to compete. This leads to a strong intention for your product design, which drives a clean and cohesive UX.

your product’s design can be more cohesive because you’re trying to do less, but better.

keeping a cohesive product experience is quite hard, and removing features is usually harder than adding them.