Usable by the Geomancer, the Terrain command gives a random effect depending on where the Geomancer casts it.
Gaia (Terrain in the RPGe translation, and Earth on the PS) is the ability of Geomancer job. It is an offensive skillset whose effects vary depending on the terrain on which the current battle is taking place. There may be several different Gaia effects that may take place in a single type of battlefield.
Mog's Dance command is essentially a terrain skill, as he learns new dances by battling in different terrains and may fail to execute dances when trying to perform on different terrain. His dance effects are random..
Geomancy (also known as Elemental) is a Geomancer's command ability. Every panel in the battlefield is designated a specific type of terrain to determine which Geomancy ability should be cast. If the caster has not learned the Geomancy ability of the grid that s/he is standing on, the command Geomancy will simply be unavailable.
Generally speaking, the damage dealt by Geomancy is relatively low, but the advantages of Geomancy include 100% hit-rate, no charge time, no MP cost, 1 grid further casting range than most magick spells, and a chance of inflicting a status ailment.
The Geomancer's Geomancy abilities can be used depending on the weather and the terrain over which the Geomancer stands. There are four abilities in total, one for each possible weather situation, and other four for terrain types, so the player can in most situations choose one out of two abilities, considering the Geomancer has mastered Geomancy.
Unlike any other job, upon being unlocked, Geomancers have two mastered abilities: Shining Flare and Nature's Embrace.