Buyback
A protocol or company using its income to purchase its own token on the market, then either destroying or holding what it buys.
Mechanically, fees accrue in some other asset, a contract or a committee swaps them for the protocol's token, and the token is either burned, reducing supply, or moved to a treasury, where supply is unchanged but ownership becomes more concentrated in the treasury. The name is borrowed from corporate share repurchases, and the differences are substantial: there is generally no legal obligation to continue, no disclosure regime governing the trades, no board with fiduciary duties, and the buyer may be a multisignature wallet controlled by a handful of people. A buyback funded by minting new tokens elsewhere in the system is circular and changes nothing about net supply.
In practice
A protocol that collects fees in stablecoins can route a share of them into a contract that swaps for its own token and sends the result to a burn address.
The common misunderstanding
A token buyback is not equivalent to a corporate share repurchase: there is no audited balance sheet behind it, no required disclosure of the trades, and holders normally have no legal claim on the funds used.