PTX - what is a CTA?

The Thread Hierarchy section of the CUDA PTX ISA document explains that, essentially, CTA means a CUDA block.

Also note that it's actually not a "Compute Thread Array", but rather a "Cooperative Thread Array" (!).

Yes, the PTX cooperative thread array is conceptually and functionally the same as a block in CUDA or a workgroup in OpenCL.

CTA is just another way of saying Threadblock Nvidia calls it CTA.