It's most definitely catenate. I understand catenate to mean chain and concatenate to be to chain together. Since "cat foo bar xyzzy" doesn't modify the files to join them in any way I don't think they're chained together.
Besides, ken & Co. aren't daft. con would be short for concatenate. :-)
Besides, ken & Co. aren't daft. con would be short for concatenate. :-)