The terrestrial kingdom is the middle degree or kingdom of the three degrees of glory in heaven. In the scriptures, the glory of the terrestrial kingdom is compared to the glory of the moon.
Linda Christensen, “My Life Is a Gift; My Life Has a Plan,” Liahona, January 2008
“Charting the Plan,” New Era, July 2006
“The Three Kingdoms of Heaven,” Friend, February 1986
“Three Degrees of Glory,” The Latter-day Saints Channel Q&A, episode 11
“The Three Kingdoms of Heaven,” Scripture Stories—Doctrine and Covenants Stories, episode 19