Thank you Klaus for bringing this up. I have the same reaction when I see the term.
Bjørn, they have been called 'supplementary lenses', 'front diopters', 'supplementary plus diopters', 'close up lenses', 'close up attachment lenses', and on and on.
In American camera store lingo, 'close up lenses', or 'close up diopters' are probably the familiar terms. In the American motion picture/television industry, the terms 'front diopter' or 'plus diopters' are more often found.
Tristin, If you refer to them as 'filters' we'll of course know what you mean, but it would be more relaxing for Dr.Schmidt's hair(and mine) if you called them 'lenses'!