In what year were tanks first used or invented?

As early as 1482 Leonardo da vinci claimed to be able to make wheeled, armored, gun (cannon actually) firing vehicles.

The armored car was introduced around 1900.

But, if you define a tank as a tracked vehicle, able to handle rugged terrain, having substantial firepower and used in actual combat, the answer is September 15, 1916.

The British used 31 of their Mark I tanks on that date in the Battle of the Somme.

If you wish to include a revolving turret in your definition the answer is the Renault FT17 introduced by the French in 1917.

Da Vinci's machine was, by today's definition, an armoured car. It was designed to run on wheels, not tracks, and was not very different from many other ideas over the centuries. It was, of course, not driven by an internal combustion engine but was man-powered.

The British and the French developed what we now call Tanks quite independently of each other and began building them in quantity at almost exactly the same time - the Spring of 1916. The British used them first, as described above. None of the first British and French Tanks had a turret. The Renault FT (not FT17) was the first operational Tank with a turret, but Tanks with a turret had been designed in Britain, Austria, and Russia, though none went into production.