Threat
A threat is something that if not dealt with will cause a player to lose. Answers deal with threats, while chump blockers delay the loss, provided the threat comes in the form of a creature. [1]
Threat reliability
Threat reliability indicates the degree to which a card has a consistent answer to threats.
The fewer weaknesses a card has, the more reliable it is. If a "new" Savannah Lions was made with "Indestructible" and no differences in cost or function it would be more reliable than the original. Generally cards with built-in weaknesses are not reliable. They can be made so, but alone they are not. Also cards able to "win" the game when they enter the battlefield or a few turns after can be considered reliable. Though it would seem logical to play with 4 cards of any superior reliability-card there are a few cards so reliable that only 1 card is needed in the deck, and usually this is supported by cards that "fetch" that card in one way or another. Cards with effects based on the flip of a coin are probably the most unreliable cards (most of the time they have a 50% chance of failure).