In the lingo of new venture development (whether in a startup or a corporate entrepreneurship endeavor), TAM refers to Total Addressable Market. Wikipedia defines TAM as follows:
"Total addressable market (TAM), or total available market, is the total market demand for a product or service, calculated in annual revenue or unit sales if 100% of available market is achieved." - Wikipedia
TAM is important because it is a measurement of market size. A venture with a low TAM will not be attractive to investors. See this article about what makes a startup fundable to see how TAM and growth rate plays into that equation.