Calenardhon was a wide green landscape in the north of Gondor, but it became depopulated during the Third Age. It was devastated by the Great Plague of Third Age 1636.
During the rule of the Steward Cirion, Gondor faced a series of attacks by the Balchoth, and Calenardhon was overrun by these evil people. Cirion sent messages for help to Gondor's old allies, including the Éothéod, to help defend Gondor.
Eorl the Young answered the call, and helped Gondor defeat the Balchoth at the Field of Celebrant. In gratitude, Cirion granted all Calenardhon, except for Anórien, to the Éothéod.
After the Éothéod settled in Calenardhon, the land became known as Rohan or the Riddermark.
Calenardhon is Sindarin, meaning Green Province (calen "green" + ardhon "province, great region").
The hypothetical Old Sindarin would likely be *Kalinarda.