Locus is a nonbasic land type used for cards that depict mana rich observing posts on Mirrodin. Created by Memnarch to obeserve his imported creatures, they were later taken over by the Praetors of New Phyrexia.
The first card to bear the type was Cloudpost in Mirrodin. It adds to your mana pool for each Locus on the battlefield. With the printing of Glimmerpost in Scars of Mirrodin deckbuilders were gifted a second playset of Locuses and, alongside the landcopying Vesuva from Time Spiral could play up to twelve in one deck. Each copy would increase the amount of mana generated by Cloudpost.