Sounds like SD it is, though finding ones that can be used again for the Nikon D90 is not as simple as it sounds. One site seems to say Nikon have never mentioned a 16Gb. 8 or just perhaps, 32. And the speed has to be brisk for that camera.
Have you considered looking in the manual ?
For all the Nikon's I've ever owned, there has always been a table detailing memory card compatibility information.