To be executed by impalement; a method used in ancient times and as recently as the 4th century AD.
Crucified also means, Extinguished, subdued, or 'Mortified' , as to "mortify the deeds of the body". (see Romans 8:13)

I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave Himself for me.
Galatians 2:20

And it came to pass, when Jesus had finished these sayings, He said unto His disciples, Ye know that after two days is the feast of the passover, and the Son of man is betrayed to be crucified.
Matthew 26:1 and 2

In days of old, the death penalty was often carried out by nailing the criminal (or innocent martyr) to a huge wooden cross (also called a tree) as punishment for crimes and left there to die. Although He was innocent of ANY wrong-doing, this was the punishment Jesus took in order to pay the penalty for us, for all of our sins.

And they that are Christ's have crucified the flesh with the affections and lusts. If we live in the Spirit, let us also walk in the Spirit.
Galatians 5:24-25