Wiktionary
n. (context cryptography English) A stream of random or pseudorandom characters that are combined with the plaintext to produce a ciphertext.
Wikipedia
In cryptography, a keystream is a stream of random or pseudorandom characters that are combined with a plaintext message to produce an encrypted message (the ciphertext).
The "characters" in the keystream can be bits, bytes, numbers or actual characters like A-Z depending on the usage case.
Usually each character in the keystream is either added, subtracted or XORed with a character in the plaintext to produce the ciphertext, using modular arithmetic.
Keystreams are used in the one-time pad cipher and in most stream ciphers. Block ciphers can also be used to produce keystreams. For instance, CTR mode is a block mode that makes a block cipher produce a keystream and thus turns the block cipher into a stream cipher.
Usage examples of "keystream".
The process of generating each character in the keystream alters T in a reversible but more or less "random" fashion.
In its general outlines (viz, an n-element permutation that is used to generate a keystream, and that slowly evolves) it is similar to a commercial system called RC4, which enjoys a complicated reputation among Secret Admirers--it seems secure, and has not been broken, but it makes us nervous because it is basically a single-rotor system, albeit a rotor that evolves.
The subroutine e generates the next keystream value whilst evolving $D.
And continuing to work through it, one letter at a time, the cards getting a little sweaty in his hands now, he eventually gets DONOTUSEP and finally loses his place while trying to generate the last keystream letter.
To encrypt, generate the same number of keystream letters as plaintext letters.
To decrypt, generate the same keystream and subtract modulo 26 from the ciphertext to recover the plaintext.
Subtract the keystream numbers from the ciphertext numbers, modulo 26.
In its general outlines (viz, an n-element permutation that is used to generate a keystream, and that slowly evolves) it is similar to a commercial system called RC4, which enjoys a complicated reputation among Secret Admirers—it seems secure, and has not been broken, but it makes us nervous because it is basically a single-rotor system, albeit a rotor that evolves.