The Templars are the Assassin order's primary rival organization in the Assassin's Creed series.
Across the historical storyline, Assassins and Templars struggle over how humanity should be governed. Assassins generally defend free will, while Templars seek order and control, often using powerful Pieces of Eden to influence societies and individuals.
The Templars are inspired by the medieval Knights Templar, but the fictional organization survives across centuries in different forms. In the modern-day storyline, Abstergo Industries serves as a major Templar front and uses technology, business, and research to pursue similar goals.
The Templars are easy to confuse with related groups. The Order of the Ancients is an earlier organization in Origins and Valhalla, while the Hidden Ones are the predecessors of the Assassins. Abstergo is the modern corporate manifestation of Templar power, not the name used for the entire historical rival organization.