Outstanding shares is the number of shares actually held by stockholders other than the company itself, calculated as issued shares minus treasury shares. It is used for voting calculations, economic ownership percentages, anti-dilution formulas, and most per-share financial metrics, making it the share count that matters most for day-to-day cap-table analysis. It is the practical denominator for ownership math, distinct from issued shares (a cumulative-issuance count) and authorized shares (a legal ceiling).
The mechanic and where outstanding shares is used: