S-box
The lookup table that makes a cipher nonlinear — and that modern designers prefer to keep out of memory.
The substitution box is the nonlinear component of a block cipher: it swaps one group of bits for another according to a fixed table. Without it the cipher would be a chain of linear operations and solvable with school algebra. The S-box supplies what Shannon called confusion.
DES has eight hand-picked 6→4-bit S-boxes. AES has a single 8→8-bit box, derived mathematically from the multiplicative inverse in a finite field — a deliberate choice, since a derived table is harder to suspect of hiding a backdoor than a hand-chosen one. The drawback of tables is that lookups take varying time depending on cache state and therefore leak information about the key, which is why newer ciphers are often built without them entirely.