What is Earth's twin planet?

Excuse me, could you please elaborate on the concept of Earth's so-called 'twin planet'? Is it a celestial body that shares similar characteristics with our own planet, such as size, distance from the Sun, or even potential habitability? Are there any scientific theories or observations that support the existence of such a twin? And if so, how does it compare to Earth in terms of its composition, orbit, and the potential for life to exist there?