Plain English Meaning
The strongest internal beam inside a wing or tail surface. It runs from root to tip and carries most of the load. On a stabilator, it's also the rod the whole surface rotates on.
Full Definition
The principal lengthwise structural member running through a wing or stabilizer that carries the bulk of the bending and lifting loads. In a stabilator, the main spar is the hinged structural backbone the entire control surface pivots around as it moves up and down.
Context Anchor
Seen in aircraft structure discussions, especially when describing how a stabilator is built and how it carries loads.
Derivation
Spar' comes from Old Norse 'sperra,' meaning a beam or rafter — the main timber holding a roof up. The aviation use carries the same idea: the main spar is the beam holding the surface together against the loads of .