In NL we call that "Jam" (pronounced: Sjem) and we make it not only from plums, but from all kinds of fruit like strawberries, cherries, and so on.
Jam (dzem in Polish) is not the same as powidla.
"Powidla" apply only when prunes are the sole ingredient. Except for (rarely) a little bit of water, nothing is added.
"Dzem" (jam), on the other hand, in addition to the fruit, contain lotsa sugar and sometimes gelatine.