A horizontal intrusive igneous body that forms between sedimentary rock layers is called a sill.

Here's why:

* Intrusive: This means the magma cooled and solidified *within* existing rock layers, rather than erupting onto the surface.

* Horizontal: Sills are generally parallel to the bedding planes of the surrounding sedimentary rock.

* Between sedimentary rock layers: The magma intrudes between the existing layers, often exploiting weaknesses or spaces in the rock.

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