Cruelty scale

A rating system devised by Andrew Plotkin to measure how difficult a particular game makes it for the player to progress past in-game obstacles:

Merciful: cannot get stuck.
Polite: can get stuck or die, but it's immediately obvious that you're stuck or dead.
Tough: can get stuck, but it's immediately obvious that you're about to do something irrevocable.
Nasty: can get stuck, but when you do something irrevocable, it's clear.
Cruel: can get stuck by doing something which isn't obviously irrevocable (even after the act).

An elaboration of the system was given by Andrew here:

Merciful: You only ever need one save file, and that only if you want to turn the computer off and go to sleep. You never need to restore to an earlier game.
Polite: You only need one save game, because if you do something fatally wrong, it's blatantly obvious and you'll know better than to save afterwards.
Tough: There are things you can do which you'll have to save before doing. But you'll think "Ah, I'd better save before I do this."
Nasty: There are things you can do which you'll have to save before doing. After you do one, you'll think "Oh, bugger, I should have saved before I did that."
Cruel: You think "I should have saved back in the third room. Now I'll have to start over."

Limitations

Cruelty is only one element of a game's general difficulty. A mostly-easy game could earn a Nasty rating, and a very difficult one might be technically Merciful. (Design norms have shifted since the scale was proposed, making unwinnable states a less common concern and other aspects of difficulty more important.)
The scale has nothing to do with how often the player is likely to get stuck. (A huge game that is Cruel in one obscure situation and Merciful otherwise is classified as Cruel.)
The system was devised before UNDO or multiple-UNDO were standard expectations; the saving behaviour described in the explanation assumes that no UNDO is available. (Modern players expect to need saves considerably less than described; for instance, a Polite game doesn't need a save if you can UNDO at all.)
The distinctions between Polite, Tough and Nasty rely upon certain things being obvious to the player. Some will really be obvious to everyone (if the game ends in failure, everyone will agree it has become unwinnable); others are only obvious to some. Because of this, the distinction between Polite, Tough and Nasty games is not always clear, and has historically been subject to some debate.
Rating a game requires comprehensive knowledge of that game, which is not always possible in works with unpublished source code (and can be difficult to discern even if source is available).

IF Cruelty Ratings diagram by Jeremy Douglass. (See also this RAIF thread.)
Unwinnable By Design at the TV Tropes Wiki. It not only extends Zarf's cruelty scale with Evil and Hell levels but it also gives examples from Infocom, Sierra, and more.