Someone or something that displaces; a person or object that removes or pushes something from its place.
From displace + -er (agent noun suffix meaning 'one who'). The -er suffix has been used since Old English to create nouns identifying agents of actions.
In Stirling engines, the 'displacer' is a cleverly simple piston that moves gas around without sealing tightly—it's a beautiful example of how technical vocabulary borrows everyday words and gives them precise mechanical meanings.
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