So special, valuable, or unusual that if you lost it you could not get another one like it. The idea is not price — it is that no substitute exists. A grandmother's ring, a photograph with no copy, a language with no speakers left, a colleague everyone relied on: all irreplaceable. It is used of people as easily as of things, and about people it is warm praise — 'she's irreplaceable' means the team could hire someone else but never get her back. It is a common word, fine in speech and writing, and slightly emotional; it usually carries either love or regret. Worth noticing: people sometimes say 'nobody is irreplaceable' as a hard-headed saying about work, which shows the word is really about how we value something, not about the facts of supply.