The total revenue opportunity if you sold to every possible customer for your product. TAM matters far less to an early DTC brand than founders think, a huge TAM means nothing if you cannot win a beachhead inside it. The useful question is not how big the market is, but how small a slice you can dominate first.
Benchmark. Year one, plan on capturing a fraction of a percent of even your beachhead (0.1-0.5% is a realistic default). Quad Lock's 2-5% capture was an outlier, not a planning assumption.