Keyed alphabet
A substitution alphabet in which a keyword decides the order of the letters.
The keyword is written first, with repeated letters dropped, and the rest of the alphabet follows in its usual order. Each plaintext letter is replaced by the letter sitting in the same position in the shuffled row as it occupies in the normal one. That makes it a monoalphabetic substitution, with the same weakness as every other one: frequency analysis exposes the pattern. A few hundred letters of ciphertext are usually enough to recover the whole alphabet, whatever the keyword was.
The ordering is hard for a human to guess but not for a machine. An alphabet of 26 letters has about 4 · 1026 permutations, which sounds like a lot until you notice that frequency analysis never has to try them one by one. Playfair also builds on a keyword, in a 5×5 grid and working on letter pairs, which holds up better against simple frequency counting.