Replacement or substitution; also used in expressions like 'かけがえのない' to mean irreplaceable or invaluable, depending on context.
彼の励ましは私にとってかけがえのないものだ。
His encouragement is irreplaceable to me.
重要な部品の掛け替え用（かけがえ）はすぐに手配した。
We quickly arranged a replacement part for the important component.
その思い出は簡単にかけがえのきくものではない。
That memory is not something that can be easily replaced.