TAM (total addressable market) is the total revenue opportunity if every potential customer in the world bought your product. SAM (serviceable addressable market) is the portion of TAM you can realistically reach given your product, geography, and channels. SOM (serviceable obtainable market) is the share of SAM you can capture, usually framed over 3 to 5 years. Investors use the three figures together to size the opportunity and to test whether a founder thinks rigorously about market.
There are two ways to calculate these numbers and only one of them earns trust. Top-down sizing starts from a published industry figure ("the global CRM market is $90 billion, we will capture 1 percent") and is almost always how founders inflate ...