bull's eye,
bull in target shooting: a score made by hitting the center of the target; "in repeated sets of five shots his numbers of bull's-eyes varied"
center of gravity,
centre of gravity the point within something at which gravity can be considered to act; in uniform gravity it is equal to the center of mass
core a bar of magnetic material (as soft iron) that passes through a coil and serves to increase the inductance of the coil
navel point,
navel a scar where the umbilical cord was attached; "you were not supposed to show your navel on television"; "they argued whether or not Adam had a navel"; "she had a tattoo just above her bellybutton"