Market definition is the activity of deciding who the product is for, precisely enough that the answer changes what gets built. The framework places it in the Focus category, next to Distribution Strategy, Product Portfolio and Product Roadmap, because all three depend on it.
Why it comes before problems get counted
Pervasiveness is a proportion, and a proportion needs a denominator. Saying that seventy per cent of the market has a problem is meaningless until the market is named. Widen the definition and the same problem becomes marginal. Narrow it and a niche annoyance becomes the defining need. This is why market definition and market problems are worked together rather than in sequence.
What a usable definition contains
Who they are, described by something that predicts behaviour rather than something that is merely available. Industry and headcount are easy to obtain and weak predictors. What matters is usually a shared situation, such as operating across more than one regulatory regime, or running a fleet the owner does not directly employ.
How many of them there are, with a stated method. A number with no method behind it is a wish, and Focus teaches estimating market size as a skill specifically because the arithmetic is where most business cases quietly fail.
What they currently do instead, which is the real competitor in most markets and is usually a spreadsheet, an agency or nothing at all.
How they buy, which is the input to distribution strategy. A segment that buys through resellers and one that buys with a corporate card need different products, not just different marketing.
Large enough for the business it has to become
The framework's wording is specific about current and future business. A segment that supports the product today but caps out below the revenue the company needs in three years is a strategic dead end that will look healthy for two of them.
The check is arithmetic and worth doing early. Take the segment size, an achievable share, a realistic price, and see what the ceiling is. If the ceiling is below the plan, either the segment is wrong, the price is wrong, or the plan is.
Choosing fewer
The most common failure is refusing to choose. Pursuing four segments with one team produces a product that is second best in all of them and a positioning statement that names none of them. Choosing one segment and serving it completely is uncomfortable precisely because it makes the size of the bet visible, which is also its main benefit.