Here's why:

* Moho is short for Mohorovičić discontinuity, named after the Croatian seismologist Andrija Mohorovičić who discovered it.

* Discontinuity in this context means a change in the physical properties of the Earth's layers.

* The Moho marks the boundary between the Earth's crust (the outermost layer) and the mantle (the layer below the crust).

* The Moho is characterized by a significant increase in seismic wave velocity. This change in velocity is due to the difference in composition and density between the crust and the mantle.

So, while the Moho isn't technically part of either the crust or mantle, it's the distinct boundary between the two.