The shares that have actually been distributed to shareholders — a subset of the authorized shares.
Issued shares are the shares that the company has actually given out to shareholders. They're a subset of authorized shares: a company might be authorized to issue 10 million shares but have only actually issued 8 million. The remaining 2 million are authorized but unissued — they don't affect ownership until someone receives them.
Ownership percentages on an "issued basis" are calculated against total issued shares. If you hold 5 million shares and 10 million are issued, you own 50% on an issued basis. This differs from a fully diluted calculation, which also includes shares that could be created through option conversions, SAFE conversions, and convertible note conversions.
Both calculations matter. Investors often care more about fully diluted percentages because they show the eventual ownership once all potential shares are created. Employees often care about issued-basis percentages because they show current voting power. Understanding which basis a percentage is calculated on is important when reading a cap table or negotiating equity.