Monoalphabetic cipher
Every plaintext letter is always replaced by the same ciphertext letter, which makes the cipher easy to break with frequency analysis.
The simplest substitution scheme: one fixed table maps each letter of the plaintext to one letter of the ciphertext for the whole message. The Caesar cipher and Atbash are monoalphabetic, as are most of the secret alphabets children invent. The weakness lies in the language itself. In English, E is the most common letter, and every E gets replaced by the same thing. Al-Kindi described frequency analysis in the 9th century, so the problem is older than most of the terms in this wiki.
The fix was to let the same letter change shape from one position to the next, see polyalphabetic cipher.