Also: PMF
The point at which the market pulls the product out of you rather than you pushing it at the market.
Deliberately fuzzy, and famously hard to define precisely. The most useful practical test is whether growth continues when you stop pushing: if you take a week off and signups stop entirely, you don't have it yet.
The trap is treating it as a binary achievement rather than a matter of degree, and within a specific segment. Fit with one narrow segment is normal and good; assuming it generalises to the adjacent one is where a lot of second-year startups stall.
Sean Ellis's survey question — what share of users would be very disappointed if the product disappeared — is the common quantitative proxy. It is a signal, not a certificate.
Project types where this term stops being vocabulary and starts being a decision you have to make.