Originally an Old English word, it became obsolete before Modern English, but fantasy author J.R.R. Tolkien revived the word in his Middle Earth series of books, most notably "The Lord of the Rings."
A mathom is any trinket or bric-a-brac, anything that its owner has no immediate use for but is unwilling to throw away.
In the Middle Earth universe, hobbits had a habit of giving presents on their birthdays instead of receiving them, and their homes usually became crowded with such presents. They attempted to solve this problem in two ways; one, by re-gifting mathoms to others, and two, by storing mathoms in museum-like houses called mathom houses. The Michel Delving Mathom-House is the most famous in the Shire.
This wiki's purpose is to become a sort of virtual mathom house. Ariel intends to fill it with information that's nice to have, regardless of any use or worth that information may have.