Planet X

Meanings

1. A hypothetical planet in the Solar system, beyond the currently known outermost planet. (The term was applied to Eris for a short while, when Pluto was still considered the ninth planet.)
2. A theoretical planet responsible for reported gravitational perturbations of Neptune's orbit. (This was initially believed to be Pluto. The reported perturbations were an error.)
3. A theoretical planet responsible for observed gravitational perturbations of Uranus' orbit. (This planet was discovered and is now known as Neptune.)

Etymology

X represents an unknown as opposed to the Roman numeral for 10 (initially, Planet X would have been the ninth planet).