Nothing-up-my-sleeve numbers
Constants chosen in a way anyone can check, so nobody can claim they hide a backdoor.
The name comes from magic: nothing hidden up the sleeve. Cryptographic algorithms use such numbers as constants. Blowfish starts from the digits of pi, and SHA-2 derives its initial values from the fractional parts of the square and cube roots of the first primes. Anyone can recompute those values from a known, dull source, so nobody could have picked them to plant a weakness.
The counterexample is NIST's Dual_EC_DRBG, a random number generator whose constants had no public justification. When suspicions of a backdoor became widely known in 2013, NIST withdrew the recommendation. The lesson: a constant you cannot explain is a constant you should not trust.