Nivation refers to a number of different processes involving the melting and refreezing of snow. It is used primarily in the fields of glaciology and geology.
First off, nivation refers to the process of snow turning into solid ice by melting and refreezing into a more compact mass. This may happen very quickly (over one summer) or slowly, spending a long time passing through various degrees of névé, depending on how much the snow melts.
Nivation may also refer to erosion and weathering to the ground under a snowpack. This weathering is due primarily to freeze/thaw erosion and chemical weathering. These processes may also encourage mass wasting, such as gelifluction and solifluction. These take many years to cause any great damage, but snow may sit over a given spot for decades or centuries, and can do quite a lot of damage. These processes are necessary to get certain periglacial formations started, for example cirques, which could not gain a foothold if nivation didn't get things started.
Nivation may also sometimes be used to refer to erosion caused by meltwater streams flowing under snowpacks, although this is stretching it a bit.