Fantail as a term covers most or all of the stern portion of the ship, including the deck; the transom, if there is one, is only the extreme terminal end of the fantail.
: Fantail: a stern shaped like a fan, extending
: over the poop on ancient ships
: Transom: The term is probably a corruption
: of Latin transtrum, a thwart, in a boat
: (Source: Encyclopaedia Britannica)