The term "petrified" means "turned to stone". While it is not really accurate, it is such a popular term that even museums and text books use it. If you want to be absolutely, technically accurate, this is permineralized wood. After the wood was buried, ground water rich in minerals flowed into the pore spaces, and started to deposit minerals. As the wood decayed away, it was slowly replaced by minerals, often preserving the grain of the wood, and in some cases, even the cell structure.