Proton
Valve's compatibility layer that lets Windows games run on Linux-based SteamOS, translating Windows API calls on the fly.
Proton is a compatibility layer, built on Wine and maintained by Valve, that translates Windows game code and graphics API calls (DirectX, in particular) into something Linux and its graphics drivers can run. It is the technology that lets a Steam Deck or other SteamOS handheld play the vast majority of Windows-only Steam games without a native Linux port.
Proton compatibility has improved enormously and now covers most single-player and many multiplayer games well, but titles using kernel-level anti-cheat (a common requirement for competitive online games) frequently do not work, since those anti-cheat systems are not designed to run under Linux. Community site ProtonDB tracks per-game compatibility ratings before you buy.
Why it matters when buying
Before buying a SteamOS handheld, check ProtonDB for the specific games you play, especially competitive multiplayer titles, since anti-cheat compatibility is the main gap. For single-player and most co-op games, Proton compatibility today is strong enough that it rarely affects the buying decision.
