Everyone has a little different twist on these definitions. The first set is from SAAMI, and the second is from Sniper Country. Notice the particular difference in thr SD deinition:
BALLISTIC COEFFICIENT An index of the manner in which a particular projectile decelerates in free flight expressed mathematically as:
c = w/id2 where: c = ballistic coefficient, w = mass, in pounds, i = coefficient of form (a.k.a. form factor), d = bullet diameter, in inches. Represents the bullet’s ability to overcome the air resistance in flight.
SECTIONAL DENSITY The ratio of bullet weight to its diameter.
Ballistic Coefficient ( BC ) - A mathematical factor representing the ratio of the sectional density of a bullet to its coefficient of form. Simply put, BC expresses a bullet’s length ( relative to diameter ) and aerodynamic shape, thus indicating its ability to overcome air resistance in flight. The higher its BC factor, the better a bullet retains its velocity and energy, and the flatter its trajectory. Most bullets have BCs between .100 and .700. Higher BCs are required for long-range shooting.
Sectional Density ( SD ) - A mathematical factor expressing the ratio of a bullet’s mass ( weight ) to its cross sectional area. SD relates a bullet’s diameter to its length ( in a given calibre, the heavier a bullet is, the longer it is ). All other factors being equal, bullets that are longer in relation to their diameter retain their velocity better, hence have flatter trajectories, hit with higher energy, and penetrate deeper, than bullets that are shorted relative to their diameter. SD does not take into account the aerodynamic shape of a bullet, which also influences velocity retention, trajectory, etc. Useful only for comparing bullets of similar shape, or when incorporating into Ballistic Coefficient factors.