Outside

In the most ancient times, before the world was created, the beings known as the Ainur dwelt within the Void, in a place described as the Timeless Halls. Through the Music of the Ainur and the power of Eru Ilúvatar, the world was brought into existence, and many of the Ainur took form and descended into it. Among these were Melkor, originally one of the most powerful of the Ainur, and another lesser being who became known as Sauron, and was originally Melkor's servant. Melkor became the first of the Dark Lords, and Sauron the second, and in this sense both of them entered the world from 'Outside'.

Our only recorded use of this term is in Tom Bombadil's recounting of his origins, quoted above, in which he certainly seems to be claiming to have seen the descent of Melkor or Sauron into the world of Arda. If literally true, this would make Tom as old as the world itself, because only a brief time passed after its creation before Melkor entered it from the Void beyond.