There are probably several ways to define a signature win for a professional golfer, but I think the most simple definition might be the victory that stamped the player as something special.
Tiger Woods won the 1997 Masters by a dozen shots, but I don't rate that as his signature win.
Phil Mickelson won the 2013 British Open, but I don't think that was his signature.
Adam Scott won the Players Championship in 2004, but that wasn't his, either.
Take a look at this list and see if you agree with my takes on signature victories.