Keystream
The pseudorandom bit sequence XORed with the plaintext in a stream cipher.
The keystream is combined with the plaintext, usually by XOR, and the result is the ciphertext. If the stream never repeats and is as long as the message, you have a one-time pad, which Shannon showed in 1949 gives perfect secrecy. The moment the same stream is used twice, the scheme falls: XORing the two ciphertexts yields the XOR of the plaintexts, and that is analysable.
That is how WEP broke. Its 24-bit initialisation vector was too short to keep RC4 keystreams unique, so collisions reused the same stream. The lesson is that the keystream is only half the design; the other half is never letting it repeat.