Mill is a verb, which means to trash a card directly from the deck (R&D or Stack) to Discard Pile (Archives or Heap).
The term is most commonly used in Netrunner to describe Noise's ability on installing a virus. eg the runner might say to the corp player "I install Imp, mill a card please".
The term mill originates from the very first CCG card with this effect: 'Millstone' from an early Magic: The Gathering set released in 1994.