Definition:
The foot of the sock can be defined generally as the whole part of the sock that covers the foot, or more specifically as the part of the foot that is knit after the gusset is finished and before the toe shaping.
In that case, the foot is distinguished from other part of the sock because there is no decreasing happening at this point, you're just knitting straight to the necessary length for your foot.
In the picture at right, the foot is the part knit in yellow.
