A scene that jumps back in time to dramatize an earlier moment relevant to the present story.
A flashback interrupts the forward timeline to play out something from before - a wound, a promise, a crime - as a scene rather than a summary. Used well it delivers backstory the reader now needs and can't get any other way, at the exact moment its meaning will land hardest.
The cost is momentum: every jump backward pauses the present-tense engine, so a flashback has to earn the stall. The frequent note is that the information could have ridden a line of dialogue or a telling detail instead of a full detour into the past.
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