掛け替えのない

Compound of 掛(か)け替(が)え (kakegae, “replacement, substitution”) + の (no, subjective case particle) + 無(な)い (nai, “nonexistent”), literally "replacement does not exist” or “no replacement.”

• (Tokyo) かけがえのない [kàkégáé nó náꜜì] (Nakadaka – [6])
• IPA^(key): [ka̠ke̞ɡa̠e̞ no̞ na̠i]

掛(か)け替(が)えのない • (kakegae no nai) 

1. (idiomatic) irreplaceable

   彼(かの)女(じょ)は私(わたし)にとって掛(か)け替(が)えのない存(そん)在(ざい)だ。

   Kanojo wa watashi ni totte kakegae no nai sonzai da.

   She's irreplaceable to me.

This term is often spelled in kana.