Authorized shares is the maximum number of shares a corporation is legally permitted to issue under its certificate of incorporation. The ceiling is set at incorporation and amendable only through a charter amendment requiring board and stockholder approval, with standard practice being to authorize well above current issuance to provide headroom for future financings, option grants, and corporate transactions. It is the legal ceiling on share issuance, distinct from issued shares (actually issued) and outstanding shares (issued and not repurchased).
The structural layers of share counts: