A megacap stock represents the shares of the world's most valuable publicly traded companies, calculated by multiplying shares outstanding by share price. While there's no official cutoff, in the US market of the 2020s, a threshold of $200 billion is often cited (per FINRA's 2022 definition, equivalent to ~$213 billion in 2024). Because definitions vary, major index providers have created their own "mega-cap" indices, such as the CRSP US Mega Cap Index (top 70% of US market cap, 179 companies as of Sept 30, 2025), the MSCI USA Mega Cap Select (min $200B), the Russell Top 50 Mega Cap (50 largest companies, ranging from $216.6B to $4.85T as of April 30, 2026), and the S&P 500 Top 10 and similar S&P indices. These indices are often tracked by ETFs like the Vanguard MGC and Invesco XLG, letting investors easily buy baskets of these giants. Ultimately, the exact cutoff differs by provider—from fixed dollar amounts to percentage-of-market rules—but all focus on the largest companies, including those with trillion-dollar valuations.