The depeg is covered in full in the entry Depegging (or Depeg), which explains why pegs break, how markets and issuers respond and the best-known cases. This page gives a short definition and explains how the word is used.
A depeg occurs when an asset that is meant to hold a fixed value against another trades noticeably above or below that value. In crypto the word is used mostly for a stablecoin that drifts away from one US dollar, but it also applies to liquid staking tokens meant to track ether and to traditional currency pegs. Small, brief moves are common and usually corrected by arbitrage. Serious depegs follow doubts about reserves, runs on redemptions or design failures: USDC fell to about 88 cents in March 2023 after its issuer disclosed funds held at the failed Silicon Valley Bank, before recovering within days, while the algorithmic stablecoin TerraUSD collapsed permanently in May 2022.
"Depeg" works as both a noun and a verb ("the coin depegged"), and "depegging" describes the process. All three forms refer to the same event, so this site keeps a single main entry.