I used the same name used in the processor's datasheets:
SCK for the ATmegq328, SCLK for ATmega16
Yes, true, but I think that's a bit of a typo.
Further on in the datasheet they refer to it as SCK:

and:

It's the same signal, and I think that, on a single page, it should be referred to in the same way, otherwise people are going to Google "what is the difference between SCLK and SCK when using SPI?".