Megaton was a unit of measurement for the power of explosions in use by humans, the Doctor, K9 and Davros.
The Master planted a fifteen-megaton bomb on Jo Grant in order to destroy the Third Doctor and UNIT; the Doctor threw it out the window and boiled out the contents in time. (TV: Terror of the Autons)
The Oracle sent a pair of two-thousand-megaton bombs were intended for the to eradicate R1C, but ended up destroying itself. K9 identified that it was impossible to defuse it. (TV: Underworld)
Davros sent a party of Daleks, carrying more than half a megaton of explosives, to hit the hull of a Movellan ship. (TV: Destiny of the Daleks)