The name the Romans gave to the place now known as Alphen aan den Rijn. It means something like 'settlement next to the white water'.
The white water they'd settled next to was not the Oude Rijn (one of the flows of the Rhine), which flows through Alphen aan den Rijn, but the Aar, a small stream connected to the river in what is now the old center of Alphen aan den Rijn.