Final answer:
A ciphertext stream is unpredictable without the key, appearing random. It's created by a stream cipher in cryptography for secure communication.
Step-by-step explanation:
A ciphertext stream is one that is unpredictable without knowledge of the input key and which has an apparently random character. The type of stream in question is created by a stream cipher in cryptography, which is designed to provide a high level of security through a sequence of pseudorandom cipher digits, which are combined with the plaintext message to produce the ciphertext. Stream ciphers are an essential tool in secure communications, ensuring that without the correct key, the output appears completely random and the original message cannot be deciphered.