A solid, tight closure of a concealed space that is placed to prevent the spread of fire and smoke through the space. In a frame wall, this typically consists of 2×4 cross-blocking between studs.
fire stop (Wikipedia)
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A firestop is a passive fire protection system made up of various components and used to seal openings and joints in a fire-resistance-rated wall or floor assembly. Penetrating cables are known as multi-cable transits (MCTs). Firestops are designed to fireproof a wall or floor assembly, impeding the spread of fire and smoke by blocking openings with fire-resistant materials.