From the White Wolf Wiki, the most complete White Wolf reference

sire: 1. a term for the father (or genetic species providing sperm) of a creature, contrasting to the female term, dam. 2. a form of address and title for a man of rank or authority, equivalent to Lord or Prince.

A sire is a vampire's creator; the one who applied the Embrace to the subject. The term is gender-neutral (thus a female is not a dam, but also a sire).

E.g. "I was Embraced by Persephone, Persephone was my sire. Persephone made me the vampire I am today."

Similarly, one's sire's sire is a grandsire.

The vampire created by a sire is the childe.