The things you kneel on are simply called kneelers by most
Catholics. When they are free standing they are sometimes called a
"Prie Dieu" from the French "prie au Dieu" or "pray to God".
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A prayer kneeler is a piece of furniture that typically consists of a low surface for kneeling upon during prayer or meditation. It can also be referred to as a prie-dieu.
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If you mean the type that one person kneels at, it's called a
"prie-dieu".