Synonyms of the word

unexpendable

• adj. Not expendable.

essential

• adj. Necessary.
• adj. Very important; of high importance.
• adj. (biology) necessary for survival but not synthesized by the organism, thus needing to be ingested.
• adj. Being in the basic form; showing its essence.
• adj. Really existing; existent.
• adj. (of a lamination of a 3-manifold) Such that each complementary region is irreducible, the boundary of…
• adj. (medicine) Idiopathic.
• n. A necessary ingredient.
• n. A fundamental ingredient.

irreplaceable

• adj. That cannot be replaced.

unreplaceable

• adj. irreplaceable.