Blowfish
The Blowfish cipher is a 64-bit block cipher, meaning that it operates on plaintext by converting it to ciphertext one eight bytes "block" at a time, where a block is some number of bytes greater than one.
For example, a 16-bit block cipher would take in the plaintext Zettelkasten and operate first on the string Zett, then on elka, and finally on sten.
Blowfish uses a variable key size between 32 and 448 bits and a Feistel Network to construct its blocks.
Warning
Note that in practice, block ciphers that are smaller than 64-bits are considered weak.
Relevant Note(s): Symmetric-Key Encryption