From The Collaborative International Dictionary of English v.0.48:
Cid \Cid\, n. [Sp., fr. Ar. seid lord.]
[1913 Webster]
1. Chief or commander; in Spanish literature, a title of Ruy
Diaz, Count of Bivar, a champion of Christianity and of
the old Spanish royalty, in the 11th century.
[1913 Webster]
2. An epic poem, which celebrates the exploits of the Spanish
national hero, Ruy Diaz.
[1913 Webster]