Generally, a Knifehand Block is any block made using a knifehand rather than a fist. So for example an Outside Block made with a knifehand would be called a Knifehand Outside Block.
When the term Knifehand Block is used alone, it usually refers to an outside block made with a single knifehand. See Outside Block for more details.
For a Single Knifehand Outward Block, the blocking knife hand is chambered palm-up at the ribs, the off-arm is chambered palm-down (i.e., thumb-in) at the collar.