Mdthbs

I've heard the term "softlock" used a lot, especially in speedrunning contexts, and I have picked up an intuitive vague definition from context. It seems to mean getting the game into a state where winning/progressing is impossible, but the game doesn't give you a game over screen or otherwise acknowledge this unwinnable state.

However, it's not clear to me that all such instances are considered softlocks. For example, some games (notably old-school adventure games) allow you to save your game and continue playing after missing your only opportunity to acquire an item or trigger an event that is required for victory. Usually this is intended by the developers as a way to troll the player or extend the gameplay time by forcing them to restart, but in some instances it may be because the player took a route that was not anticipated by the developers and sequence-broke the game. I've also heard of saving (or auto-saving) immediately before an impending death cited as an example of a softlock. Yet another example is cases where some or all of the controls become unresponsive, or a critical menu screen becomes inaccessible, while the game continues to run normally otherwise. And of course there's cases where you simply fall into an inescapable pit, get stuck in a wall, or are otherwise immobilized without dying.

The above examples represent a wide variety of "unwinnable states", but which of them are considered softlocks? Does it depend on triggering glitches or other behavior that the developers didn't intend? Does it depend on whether or not loading your saved game fixes it, or whether it can only be fixed by restarting from the beginning of the game? Are there any other considerations I've missed that distinguish a softlock from some other kind of unwinnable state? What is the definition of a softlock?

A softlock is an instance where game remains in a playable state, but progressing further or past a certain point becomes impossible. It's an event that can happen to speedrunners where triggers and events are hit out of order, causing the games script to break completely, thus causing the progression breaker.

Contrast this with a hardlock, which locks up the game to the point where command inputs do nothing, and the game becomes unresponsive.

With the examples you presented:

For example, some games (notably old-school adventure games) allow you to save your game and continue playing after missing your only opportunity to acquire an item or trigger an event that is required for victory.

Defeat does not constitute a softlock -- defeat is just another ending for the game. A bad ending doesn't inhibit your gameplay commands nor does it render the game unplayable, it simply means you lost.

I've also heard of saving (or auto-saving) immediately before an impending death cited as an example of a softlock.

This is a player-induced softlock, not a game softlock. The player has put them into a state where they cannot progress further, so yes, this is a softlock.

Yet another example is cases where some or all of the controls become unresponsive, or a critical menu screen becomes inaccessible, while the game continues to run normally otherwise.

If the game is inhibiting players from inputting commands or accessing a critical game function that would otherwise allow the player to progress, this is a hardlock, not a softlock.

And of course there's cases where you simply fall into an inescapable pit, get stuck in a wall, or are otherwise immobilized without dying.

This is a softlock. Normally developers have teleport triggers that reset a player back to a playable position in the world in the event they fall off a pit and such (or just kill the player, forcing them to restart at a previous checkpoint).