What is the difference between witch and Which in The Phantom Tollbooth?

In the Phantom
Tollbooth, the Not-so-Wicked Which is a character also known as Faintly Macabre or Aunt Faintly.
When Milo is falsely accused and arrested by Officer Shrift, he is warned about the
"Which" in the dungeon, who he understands to be a "witch." However, once he
reaches the dungeon, she introduces herself and explains her story. She was once responsible for
regulating the use of language in Dictionopolis, but her standards became so specific and she
hoarded so many words for herself that many people become afraid to speak in public at all. The
king put her in jail for life, because hoarding words was hurting the economy of Dictionopolis,
and people regained the ability to speak freely. The Which says that once Rhyme and Reason
return to Dictionopolis, however, she will be freed from the dungeon. There isn't actually a
witch in the book, and references to a witch result from Milo misunderstanding Officer Shrift
telling him about the Which.