n

The Mean Motion (n) is defined as the number of orbits (revolutions) the satellite completes about the Earth in exactly 24 hours (one solar day).

Theoretically, the value of the Mean Motion can be anywhere between 0 and 17 orbits per solar day.

The satellite orbit's semi-major axis (a) can be found by using the following equation:

a³ = GM / (2pn)²

where a = the Semi-Major Axis of the satellite's orbit;
          G = the Gravitational Constant;
          M = the Mass of the Earth; and
          n = the Mean Motion of the satellite's orbit.

For the ISS TLE:

"15.74668600" = 15.74668600 orbits per solar day (a period of 91 minutes, 27 seconds per orbit).