Freezing over; the process by which water surfaces become covered with ice (or the ice formed). Often used intransitively to describe rivers or lakes freezing.
今朝は川が結氷していて渡れなかった。
This morning the river was frozen over and couldn't be crossed.
結氷によって船の運航が遅れた。
Ship operations were delayed due to ice forming.
池の表面に薄い結氷が見える。
You can see a thin layer of ice on the surface of the pond.