A market structure shift (MSS) is a candle close beyond the most recent confirmed swing point against the prevailing direction — below the last higher low in an uptrend, or above the last lower high in a downtrend. The mechanical reading: swings are confirmed with a fractal lookback (commonly three bars on each side), and the shift triggers on a close, not a wick, through that level.
In ICT-style trading, an MSS is the moment the trend’s defining pattern breaks: an uptrend exists while price prints higher highs and higher lows, so closing under the last higher low is the first objective evidence of reversal. It is often required as confirmation after a liquidity sweep before taking the reversal trade.
Terminology is unstandardized — many traders call the same event a change of character (CHoCH), and lookback settings change which swings count — so the signal’s meaning depends entirely on the exact definition used.