Indistinguishable from random (IND)2 min read - Text Only
A cryptographically-suitable key is a sequence of bits that are indistinguishable from random. In academic literature, it is typically contracted to IND. Others may say that it is "uniformly random" as it was selected from a uniform distribution of possible configurations.
If there is a bias in which states are selected, then it is not uniformly random. Likewise, if there is a bias when it is encoded to binary, then it is not uniformly random in binary.
In practical terms an IND bit string cannot be meaningfully analyzed. It is also impractical to guess or brute force when it is sufficiently long.
You may see names like AES-128 or SHA-256 and wonder what that's about. The names refer to how many bits there are in their configuration or design.
Asymmetric keys and asymmetric computations have distinctive patterns, which makes them unsuitable for use in symmetric cryptography like AES-256. See Why some cryptographic keys are much smaller than others (archived) by CloudFlare. A distinctive pattern makes it not IND!