In addition to the definition provided by Webster 1913, bore also has two definitions pertaining to internal combustion engines. The bore is the diameter of the cylinder(s). It is usually given in inches here in the U.S. Most automotive engines have bores in the 3 to 4 inch range. In Otto Cycle engines (gasoline engines) the larger the bore, the poorer the combustion efficiency and quality. Detonation is increased, which increases the fuel octane requirement. The good thing about having a large bore is that the displacement is proportional to the square of the bore. So doubling the bore quadruples the displacement, tripling it increases the displacement 9-fold, and so on.
In addition, since an increased stroke puts extra stress on the connecting rods, increasing the bore instead of the stroke allows extra displacement without extra stresses.
Bore as a verb means to enlarge the cylinder diameter in an engine. The amount increased is small (usually 0.030-0.050 inches), which increases total displacement by a few cubic inches. Although the horsepower gains from boring are minimal, it is still popular.