Depeg
When a stablecoin trades away from the value it is designed to track, such as below one dollar.
A deviation can come from doubt about the reserves, from friction in the redemption channel, from a single venue running short of one side of the pair, or from the outright failure of the stabilizing mechanism. Because redemption is usually restricted to approved clients, the exchange price and the redemption value are separate things, and the two reconnect only as fast as authorized parties can arbitrage between them. Small, brief deviations occur regularly and resolve; large, sustained ones have historically indicated either a genuine reserve problem or a design that cannot recover. Deviation is measured as the distance from par and is most informative when read alongside redemption activity and changes in total supply.
In practice
A stablecoin can trade below par on one venue where sellers outnumber buyers while trading at par elsewhere, which reflects that venue's liquidity rather than the issuer's reserves.
The common misunderstanding
A brief price deviation on one exchange is not by itself evidence of insolvency; it may reflect local liquidity rather than the state of the reserves.