The Templar Order is the secret organization opposed to the Assassins throughout Assassin's Creed. The two groups represent competing visions of human society: the Templars generally seek order through control, while the Assassins defend freedom of choice.
The rivalry is fictionalized through the franchise's historical settings. Templar characters and organizations appear in medieval Jerusalem, Renaissance Italy, colonial America, the Caribbean, Victorian London, and many other periods. Their plans often involve Pieces of Eden, powerful artifacts linked to the ancient Isu civilization.
Abstergo Industries is a related modern organization, but it is not the historical Templar Order itself. In the modern-day storyline, Abstergo operates as a corporate successor and public-facing institution connected to the Templar cause.
The term “Templar” draws on the real medieval Knights Templar, a Catholic military order founded in the early 12th century. Assassin's Creed uses that historical name while creating a fictional continuation and conspiracy around it.