Keyword cipher
A substitution cipher in which a keyword decides how the alphabet is shuffled.
Write the keyword first, leaving out repeated letters, then fill in the rest of the alphabet in order. The resulting sequence serves as a substitution alphabet against the plain one. The result is a monoalphabetic cipher, just as exposed to frequency analysis as Caesar, but with a far larger key space: a keyword can yield only a fraction of the 26! possible alphabets, yet that fraction dwarfs Caesar's 25.
It was a staple of teaching and of newspaper cryptogram puzzles for a long time. It shows why a big key space is not enough. The permutations of 26 letters are huge, but the letter frequencies of the language still leak through, and that is exactly what the attacker uses.