Entropy
The amount of real randomness behind a wallet seed — the lower the entropy, the easier the keys are to guess.
Entropy is a measure of unpredictability. A Bitcoin wallet seed is only as secure as the entropy used to generate it: a standard 12-word seed is designed to carry 128 bits of entropy, meaning an attacker would need to try an astronomically large number of combinations to guess it.
If a device’s random-number generator is flawed — as happened with certain Coldcard firmware versions, which produced seeds with as little as ~72 bits of entropy instead of 128 — the resulting keys become dramatically easier to brute-force, even though the seed still “looks” like a normal 12 or 24-word phrase.