Definition. Market Cap (network value) is the product of the current supply and the current USD price of BTC.

Technical. For BTC the current supply is computed as the sum of all unspent transaction outputs, including non-standard outputs. For other assets, the circulating supply across all blockchains the asset is issued on is used, and amounts not considered circulating, such as locked or treasury holdings, are excluded. What counts as circulating is not standardized, so other sources can show a different figure.