TREE: 1. δένδρον [(dendron) Strongs No. 1186], a tree, a living, growing tree;
2. ξύλον [(xulon) Strongs No. 3586], wood, i.e. for fuel, timber; then, anything made of wood; here, a piece of timber, a wooden stake
(a) [Used here for the σταυρός on which Jesus was crucified. Both words disagree with the modern idea of a cross, with which we have become familiarised by pictures. The σταυρός was simply an upright pale or stake to which the Romans nailed those who were thus said to be crucified, Σταύρου, merely means to drive stakes. It never means two pieces of wood joining each other at any angle. Even the Latin word crux means a mere stake. The initial letter X, (chi) of Χριστός, (Christ) was anciently used for His name, * until it was displaced by the T, the initial of the Pagan God Tammuz, about the end of cent. iv.]
Strongs numbering system. E.W. Bullinger definition.