Webster's Dictionary is any of the English language dictionaries edited in the early 19th century by American lexicographer Noah Webster, as well as numerous related or unrelated dictionaries that have adopted the Webster's name in honor.

What kind of word is which?

The word which is a pronoun and a determiner. Definition: Used referring to something previously mentioned when introducing a clause giving further information. Examples: "We had to go to a conference in Vienna, which ended on Friday."

What do you mean by which is which?

—used to say that one is unsure about the identity of each member of a group One of the twins is named John and the other William, but I don't know which is which?