Treasury stock is shares the company has issued and subsequently repurchased that the company itself now holds in a corporate treasury account. The shares are neither outstanding (held by parties other than the company) nor canceled (retired from the issued count), carrying no voting rights, no dividend rights, and not included in EPS or per-share metric denominators, with the company able to either reissue or formally retire them by board action. It is the structural category for shares the company has bought back but not yet canceled or reissued.
The mechanic of treasury stock: