Earth/ɜːrθ/ Definition 1 of 6Pro Tip 1/3Astronomical Object Used to refer specifically to the planet we inhabit, not just any planet. Scientists search space for conditions like those on earth. Pro Tip 2/3Not Always Literal Can be used figuratively to mean the human world or all of nature. We must protect the earth for future generations. Pro Tip 3/3Capitalization Varies Capitalized as 'Earth' when specifically discussing the planet, lowercase otherwise. Astronauts saw Earth from space.