Retired test pilot, CSA astronaut and ISS Commander Chris Hadfield's new Master Class advertisement video Chris Hadfield Teaches Space Exploration - Official Trailer - MasterClass is heavily edited and I think mixes the explanations for several things together.
You have to push your lungs forward through the drag of the atmosphere. Crushed into your chair — you’re like a leaf in a hurricane. The reason is $D = 1/2 \rho v^2 S$. Sixteen times the speed of sound. As you accelerate harder and harder, that light blue Florida sky starts to get darker and darker, and then suddenly, black — and the engines shut off, and you’re weightless.
If the equation $D = 1/2 \rho v^2 S$ is for air resistance (drag), then $S$ might be $C_D A$ where $C_D$ is a drag coefficient and $A$ is an effective cross-sectional area.
$S$ doesn't show up in that Wikipedia article or a few other sites I checked, but is it nonetheless a standard variable or parameter used in spaceflight? If so, what's it called and how is it defined mathematically, and determined experimentally and theoretically?