Regelation is the process by which ice melting that is melting under applied pressure refreeze when the pressure is reduced. Gelation, to form a gel, regelation to form a gel again. Gel, here, being a synonym for freezing.
A simple experiment to demonstrate this phenomenon involves looping a fine wire around a block of ice, with a heavy weight attached to the wire and hanging freely below. The pressure exerted on the ice by the weight of the weight slowly melts locally where the wire is pressing on the ice. The wire can thus travel downwards through the ice and pass through the entire block. The ice above the wire, released from the pressure of the wire is able to refreeze.
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The experiment works best if the ice is at ?10 degrees Celsius or colder and using a thin copper wire, i.e. a metal with high thermal conductivity. This is necessary to allow the latent heat of fusion (freezing) from the upper side of the wire to be transferred to the underside and so act as latent heat of melting. The exact details of how the processes work are not yet fully understood. It is worth noting that the phenomenon only occurs in water and other substances that expand, rather than contract, when they freeze.*
The regelation phenomenon was first described by 19th Century polymath scientist Michael Faraday.
*The fact that water expands when it freezes means that icebergs are less dense than liquid seawater and so can float. This is also related to how ponds and lakes freeze from the top down and so can stay liquid below an icy surface allowing life to survive within the cold liquid beneath.