It comes from the Greek name Αλέξανδρος (Alexandhros), which means ""defender of people"," defender of man", from Greek αλέξω (alexo) "to defend, help" and ανήρ (aner) "man" (genitive ανδρός, "andhros").
(This name was most famously borne by Alexander the Great, King of Macedon. In the 4th century BC he built a huge empire out of Greece, Egypt, Persia, and parts of India. The name was later used by emperors of Russia, kings of Scotland and Yugoslavia, and eight popes.)
Feminine: Alexandra, Alessandra, Alexa, Alexis, etc.