A land mine was an explosive device triggered by pressure, usually when it was stepped on.
While on Skaro near the end of the Thousand Year War, the Fourth Doctor stepped on a land mine and was forced to stay still as he expected it to detonate if he took his foot away. Harry Sullivan placed rocks beneath it to steady it and the Doctor moved slowly off it. (TV: Genesis of the Daleks)
While travelling through the Tower of Rassilon in the Death Zone, the Master tricked several Cybermen to their deaths by having them walk across a trap resembling a giant chessboard, which killed anyone attempting to cross it except those who correctly applied pi. He compared this to humans driving sheep over land mines during wartime; Tegan Jovanka argued with this comparison, saying that "this minefield is still just as dangerous". (TV: The Five Doctors)
In the 19th century, near Killingworth, the Rani hid in a wood special land mines, able to transform animal beings in vegetable beings. They were supposed to hit the Sixth Doctor, but eventually its victims were humans like Luke Ward, who was transformed in a near-sentient tree. The Rani remarked that he and any other victims were better off, as trees had 4 times the life expectancy of a human. (TV: The Mark of the Rani)