Market validation tests whether demand exists beyond the builder's enthusiasm. Evidence can include current spending, recurring workarounds, interviews tied to behavior, sign-ups, trials, purchases, retention, or growth through a reachable channel.
An established product category reduces uncertainty about the broad need, but not about a new entrant. A smaller product still must prove that its differentiation and distribution are strong enough to attract and retain a viable audience.
ELI5
Market validation checks whether enough people actually want a product, not merely whether the idea sounds appealing. Strong evidence comes from what potential customers do and spend, not only from polite opinions.
For example, existing task-management products show that people pay for the category. A new task app must still show that customers care about its simpler collaboration feature and can be reached economically.
