Titan is a Grimoire card from the original Destiny (Destiny 1), found under Guardian → Classes. Grimoire cards held most of the game's lore and were read on Bungie.net and in the companion app, not in the game itself.
Card text
Titans are warriors - heroic defenders of the Light, channeling the gifts of the Traveler to wage war on the Darkness. Steadfast and sure, Titans face any challenge head-on, blunt force instruments of the Traveler's will.
Grimoire text written by Bungie, shown as it appears in the Destiny 1 game files.