Thayer's Greek Lexicon

 1. an upright stake, esp. a pointed one
2. a cross
  1) a well known instrument of most cruel and ignominious
punishment, borrowed by the Greeks and Romans from the
Phoenicians; to it were affixed among the Romans, down to the
time of Constantine the Great, the guiltiest criminals,
particularly the basest slaves, robbers, the authors and
abetters of insurrections, and occasionally in the provinces,
at the arbitrary pleasure of the governors, upright and
peaceable men also, and even Roman citizens themselves
  2) the crucifixion which Christ underwent