Footprints were marks made by feet.
Susan Foreman discovered giant footprints in the snow on the Plain of Pamir. (TV: The Edge of Destruction)
On Xeros, the Doctor and his companions appeared to not leave footprints in the dust as they had jumped a time track. The Moroks later discovered their footprints. (TV: The Space Museum)
At Emmett's Electronics, Benton discovers K1's giant footprint. (TV: Robot)
On one planet, Xoanon's projections left footprints, even though they were images of the Doctor's face. (TV: The Face of Evil)
On Skaro, a Dalek tracked the Doctor by his footprints. (TV: Destiny of the Daleks)
The Tenth Doctor noted that a footprint did not look like a boot. (TV: Army of Ghosts)
In The Library, the Doctor compared a data ghost to a footprint on the beach with the tide coming in. (TV: Silence in the Library)