the cross: the upright wooden stake with a crossbeam on which the Romans executed condemned people, above all the one Jesus was crucified on, and by extension his crucifixion itself as the saving heart of the Christian message
In the ancient world this word named a real and dreaded object: the wooden frame on which Rome put criminals, rebels, and runaway slaves to a slow public death. It was a word of horror and disgrace long before it became a symbol of faith. Every New Testament use grows out of that grim literal meaning.
From the physical object the word stretches in two directions. Jesus turns it into a picture of discipleship: to take up your cross is to accept rejection and even death for his sake, following him all the way. Paul then makes the cross a kind of shorthand for the whole saving event, so that the cross can stand for Christ's death and the good news about it, what he calls the word of the cross.
These are not separate words but one word seen at different distances. Up close it is the timber Jesus was nailed to. A step back, it is the shape of a follower's whole life. Further back still, it names the single act in which, for the New Testament writers, God reconciled the world to himself.