The Propagation

Kind of path followed by a radio wave to reach its destination.
There are many types of propagation:

earth wave, when receiver can see the transmitter or otherwise it is not so distant from a direct view
space wave
sporadic E
tropospheric duct
MS
TEP

With the exception of earth wave or repeater conversations, the extreme mutability of propagation make the contacts absolutely unreliable... but this part of the charm of ham radio.